Follow EVERY step, there is a reason for every step. Having said that, the ROM I build using the latest Gargoyle code I check out a around a week ago seems to have problem for RT305x.
Even if you manage to get the bin, it may not work correctly. This is what I have found and I don't know what the problem is.
You can also select ALL (default for menuconfig, tick all for kernel_menuconfig) rt305x and I think you would see bin files for some models appear and others won't due to unknown reasons.
Have fun and don't ask me how to fix it because I don't know.
I would be interested to know what you did to eventually make it work though
Tip: look at the build log to see what the error is and try to fix it.
Gargoyle building turns logging on by default (log on console screen only) but Openwrt doesn't. Use command
Make V=99 2>&1 | tee build.log
under custom-src to get the log in build.log so you can go through it and find the problem.