Not GP but I use a similar setup. I have an "extended" US keyboard layout where AltGr+(location of character on native keyboard layout) types the character. For example, AltGr+[ is "ü". I find this more convenient than the compose-like approaches suggested elsewhere in this thread.

I use it because programming language syntax is mostly ergonomic only on US-like layouts, e.g. typing "{" causes a lot of strain if you have to do it on every other line.