I think he meant to merge with IC2, not add to the modlist along with IC2.
(also the BC power conversion is broken atm afaik)
1. I am neither Greg nor Player/Thunderdark but I imagine it would be a nightmare to add to the current experimental build. The mods are massively different, and Greg literally just finished the biggest part of his total rewrite.
2. It would most likely not update the maps unless it implemented some form of retrogen (where world generation spawns after the world is created) which would be god awful, because Gregtech ore generation is monumentally different from IC2's
3.No Gregtech machines run off EU (after version 5.00) except the transformers. Yes it is easy to convert afaik, but not only have I not tried, but I imagine it would be less than fun. Anyway, most IC2 machines are replaced by Gregtech when Gregtech is installed.
Essentially, don't expect it, it would really suck for the developers, your maps would be destroyed, and don't worry about them being cousins. It is what enables choice in the modding community.
IC2 machines can run off Greg EU directly.
If you wish to convert from IC2 EU to Greg EU, use GT transformers.