I wasn't able to run GateOne. (I tried the online demo - not there; building from source - a Python stacktrace; and I looked for prebuilt binaries.)

It looks like most of the code (including the escape sequence handling) is in Python. I gather most of the logic is server-side. While DomTerm has quite a bit of server-side code (basically the lws-term directory, written in C), the majority is JavaScript.