I did this in my last world. Your mileage may vary!
1. Optional: establish TE pulverizer, to make use of GregTech's bonus yield recipes (especially with recipe overloading allowed in the TE config)
2. Establish basic IC2 infrastructure: extractor, compressor, batbox, geothermal generator(s)
3. Find lots of resources, including bauxite
4. Establish Railcraft blast furnace, it can smelt aluminium dust, and you can do lots of things with steel in GregTech
5. Build an MFE or MFSU, since you will need buffer storage to avoid GregTech machines stalling on you in the middle of work
6. In any order you wish:
6.a. Upgrade extractor to industrial centrifuge
6.b. Upgrade geothermal generator(s) to thermal generator(s)
6.c. Build industrial electrolyzer
6.d. Build plate bending machine
7. Build chemical reactor and centrifuge rubber wood for carbon cells. Combine with hydrogen cells from electrolyzing bauxite to form methane
8. Build gas turbine to consume methane
9. Build industrial blast furnace
10. Build rock cutter
By this point you are firmly in GregTech's midgame.
