This is an easy one. All you need is an .int reference.
First, make a new .int file in WordPad or something with your main .u file's name. (Or duplicate an existing .int, rename it accordingly, and then open it and delete all the text from it.)
Now that we have one ready, start with this line:
This tells UT that the information under this heading can be read by any script in the game and used when necessary.
Now, the next line is from Botpack.int of UT version 436, but it should illustrate things quite nicely.
Code:
Object=(Name=BotPack.DeathMatchPlus,Class=Class,MetaClass=Botpack.TournamentGameInfo)
Replace BotPack.DeathMatchPlus with the package and class name for your gametype.
Class=Class just means that this object is a class script, rather than a sound or a texture or some other such thing, so there's no need to change this.
MetaClass=BotPack.TournamentGameInfo tells UT what kind of class this is. If you've coded your gametype for UT2003 or UT2004, then this will probably be something entirely different. Look in your game's .int files for lines like this to see what metaclass the gametypes use.
After filling in the information appropriately, just save the .int file, make sure it's in your System folder, and it should work properly.