How are you launching Minecraft? If you use MultiMC or other custom launchers they might make this easier for you... otherwise, you'll have to start using a manual call to the .jar file, passing the argument -XX:MaxPermSize=128m
This is however only a band-aid (note the "-XX:" part denoting a highly nonstandard argument), and having to use it clearly indicates the in-development state of that IC2 build. Running out of permgen space is something that might happen if you load many dozens of mods that all interact and compete for system resources, but no single mod should be consuming any significant part of the standard 64 MB. You can expect IC2 to be much more conservative in that regard when they prepare to release, but for now, it's a dev build for a reason.
On the server I started last weekend, I'm running build #307 with GregTech 3.05g. So far, neither me nor my 4 other players have encountered issues with it. If you don't pair it with something else that absolutely requires a newer build, I'd find one like that that simply works and then don't worry about keeping up to date for the next few weeks.