This minetweaker script should make biogas burn in gregtech turbines (keeping mind you'll get less total EU, this just saves you space and maybe lag vs whole fields of semifluids). I haven't actually tested it yet though.
I actually would love it if it were registered with some sort of RF engine (or with steam boilers, given that gregtech can already use normal everyday steam). Conversion via electrical engines is limited (only 20RF/t without losses), and I like having a single power source. Sadly Minetweaker's hooks don't get into any engines or dynamos right now (ignore the wiki saying combustion engines, buildcraft support was bugging out and minetweaker pulled it).