Well, I'm no expert, but I can tell you this:
There's a lot of stuff that goes into making a Jailbreak map. Making all the games requires a heck-load of logic. Moving doors, trigger_pushes, buttons, relays, text, and probably some custom textures... Judging by the fact that you've never posted a map to this site, I would suggest that you hold off on custom gamemodes until you've familiarized yourself with general mapping, logic outputs, and the like. However, if you do feel like you have to make a Jailbreak map, a program called BSPsrc allows you to decompile maps, so if you can locate a Jailbreak map and run it through that, you might have a basic idea of what you're doing by flying around the map a bit.
Otherwise, you should probably get used to using Hammer before venturing into the Logic Land that is Custom Gamemodes.