Went to try and make a test map in GTK Radiant 1.6.6 and when I tried using the Fast Test feature in bsp, it said "Connection Closed" after loading and didn't start. I was watching a guy on YouTube who successfully did this, but he had ioquake3. I finally managed to playtest it in Quake 3 though.

Also having a hard time making terrain. I remember making terrain in GTK radiant years ago, but now I forgot how I did it.

For the "Fast Test" Bsp menu option you may have a error in the command list. Personally I do not use the menu option at all and have a custom build script that does a number of things I want.

Open up baseq3/scripts/default_project.proj in a text editor. That is where the Bspc menu option is "compiled" from. It is an XML file which you can edit. Add, remove or change the compile features to suit you. Best to make a backup first.

You could make your own "project" or "game" with build options too. Check out the installs directory. Make a copy of the Q3Pack. It has been a long time since I modified GtkRadiant like this, so I can not remember which files are important (or need to be edited). I just use my customer build script instead now :]

Not sure if I understand that, but there are some things I want to learn how to do in the editor that I'm having trouble finding sifting through the manual:
1-How do I select a single face of a brush? I want to make CTF banners, but I know I need to make the other sides set to "shader not found" so only the one side has a banner.
2-How do I create my own terrain brushes? I took a terrain brush from another map as the base for the terrain in a test map I'm making, but when putting terrain pieces together I find myself intersecting the brushes to prevent leaks (I tried making them meet, but even with "snap to grid" turned off they won't)

You can select single brush face or faces by holding down at the same time the shift+ctrl and then click on the brushface(s).
For non visible faces on brushes like walls and floors, use common/caulk, and brushes like flags, use common/skip.
You can make terrain in raidant by using the plugin called gtkgensurf/ground surface.
I'm always preferred doing it "manually" with the old "triangle" style terrain. Make flat 3 side brushes (make a single brush, and keep it selected, then press ctrl+3), then put them together. Select all the brushes, disable the scaling along the X and Y axis, and then press V. You will see green dots. You can now increase, or decrease their heights. That's how the old style terrain works "manually"...
Here is some tutorials for terrain blending and brush work terrain by Simonoc:
www.simonoc.com/pages/articles.htm

Thanks Martinus. previously I was able to test my map without putting a pk3 file together, but for some reason now Quake 3 says the map isn't there. I just hope when I'm done making my map it doesn't tell me it won't run because of some stupid technicality.

If you can not load your map unless it is in a PK3, then you have a configuration error of some kind. Either the BSP is not in the baseq3/maps directory, or you are not starting with +set sv_pure 0 or maybe devmap "mapname"

Figured out what the problem was. I forgot to choose the "Q3Map2: (single bsp) -meta" option on the Bsp list to create the .bsp in the first place.

Edited 3 days after the original posting.