Okay then, where to start?
I'll start off with a common issue; optimisation. The level is currently all one zone, and the layout gives plenty of opportunity to zone off many sections to improve frame rate. It isn't atrocious at the moment, but the layout gives you the opportunity to get this level running at a significantly higher rate. You also have no antiportals at all. You mentioned that you used antiportals in the SE version of Gordian, but they certainly weren't applied in the standard way. I should be able to open up the find tool, type in "anti" and have a list of antiportal actors. That didn't happen in Gordian, so I would like to know what method you used to incorporate antiportals into that level. Did you create brushes and use the "Add Antiportal" button? In any case, this beta is currently also devoid of antiportals and again, to improve frame rate further. Your terrain is very wasteful. Given the small area that it actually appears in, you need to change the dimensions. Try and keep the terrain just slightly larger than the area in which it appears (i.e. slightly wider than the corridors and rooms that you see it in). It may be neccessary to use more than one terrain info to get the best optimisation, and don't be afraid of zoning across terrain. As long as you have the terrainzone=true in each zone, the terrain will be visible. To assist you with optimisation, are you familiar with the "rmode" console command? When you're in the level, bring down your console and type "rmode 1" without the quotes, and this will switch you to wire frame mode. You can now see all the static meshes that the engine is drawing (currently all of them), as well as the wasteful terrain. This is what the engine is actually drawing. This view is very helpful for seeing where antiportals should be placed, and if they're doing the job you thought they should. To switch back to normal view type "rmode 5", which is the normal lit version. There are other rmodes which are equivalent to the views in the 3D window of the editor (i.e. unlit, zone/portal, lighting, BSP cuts). The numbers don't neccessarily correspond to the views, but each has it's uses.
Next is the lighting. You need to do a lot more work with all of your lighting. Both the torches and the electric lamps need adjustments. For the lamps, you need to move you light actors much closer to the flame emitters, increase their brightness to somewhere between 160 and 180+ and reduce their radius to a maximum of 16, but preferably less (8-10). You then need to duplicate each light actor, drop the brightness to 24-48, but increase the radius to something like 32. This gives you the hotspots, but also ensures a pseudo-ambient light effect. With your blue lamps, a similar approach is needed, however the static mesh lights are not currently convincing as light sources. This is due to their vertex lit nature, so placing a single light in front of them simply won't evenly illuminate them. You'll need to play with several light actors around each mesh to get the desired effect. It might not seem very intuative, but the effort is worth it. There is a light mesh from one of the Egyptian mesh packages that requires something like four light actors to get it to look as though it's actually emitting light. The level overall is rather too dark, and picking out players can be a real chore in several locations. Perhaps study the original a bit more in this respect.
Still on lighting, but incorporating it into discussion on theme. The original level uses a tech gothic theme which worked well. Your current theme can't seem to decide betwen industrial and gothic, and suffers for it. I think you need to bite the bullet and decide betwen using lamps and using torches. For my money you'd be far better off sticking with the lamps, because you can still vary their light colour, whereas torches need that orange/yellow hue. Also, I think you should think about you're texturing some more. I think the level would work better as metal and stone, rather than stone and wood with a dash of metal. This would be more in keeping with the original tech gothic theme, but should still work well if you go for a straight industrial theme. I didn't like the pool of water on the first floor of the level. It looked erouneous. With no water source, it really looked forced and contrived. Such a device would be better on the ground floor set amongst your terrain.
milb is right about adding detail (and there are enough Campgrounds remakes, but you've come this far
). The level really needs some extra detail throughout. This doesn't have to get in the way of players. As milb suggested, having cutaways in the walls filled with meshes adds a lot of detail without encroaching into the play space. Similarly, you have an abundance of ceiling space where players will never venture, so don't be afraid to add plenty of overhead piping and other suitable mesh work to reinforce the industrial aspects of the level. Initially I did like the boxes and ramp you've used near the armour pickup in lieu of the normal stairs, however during play it became clear that this setup restricts players and makes their movement far more predictable and precarious in this area and I'm not convinced this is a good thing. Consider reinstating the wide stairs there.
Finally, weapon placement is always going to be open to interpretation. I am unfamiliar with the "G1E" version of the level. I only ever played the stock version and the Pro version. I'd recommend you stick to one of these unless the G1E is considered a superior version for some reason. Given that I am unfamiliar with the weapon placement for the version you are basing this level on, I'll just give my opinion on the equivalent weapons in UT2004. The Plasma Gun is obviously replaced with the Link Gun. The Rocket launcher is a no brainer, but after that it gets a little more tricky and possibly less obvious. The Shotgun should be replaced with the Flak Cannon. The Lightning Gun should be replaced with the Minigun. The Railgun is the most open to interpretation as 2k4 has two hitscan weapons. Although the Lightning Gun is perhaps the best analogy, but if it's a choice between the two, I'd recommend using the Shock Rifle. Whilst technically not the single hit kill that the Railgun is capable of, I think the weapon is more versatile than the Lightning Gun whilst retaining the hitscan considerations. There is the possibility of using the Super Shock Rifle, as found in Instagib, however this is generally frowned upon by most players and level designers alike, and unless you want to go out of your way to stir up some controversy I'd suggest ignoring that option!
I'm most familiar with the standard version of the level, however the Pro version was produced after extensive consultation with hardcore players, so my recommendation would be to duplicate the Pro version item placement. The other point on weapon placement is that it might be worth having a play with weapon placement by removing all the weapons, ignoring any previous layout and have a think yourself about where the UT2k4 weapons would be best placed. Things like the flak are generally more advantageous up high (like where you have the Bio Rifle now, or the Rocket Launcher at the top of the jump. There are lots of options if you take this path, and then you would be applying UT2k4 specific logic to the level, rather than attempting to use analogies for the weapons. Just a thought.
I'll be interested to see what improvements you can make before your next beta. Pathing would be good, even if it's just rudimentary. If you have any questions on anything I've mentioned just ask, or PM me.
Good luck.