Is Aero Precision M5 based on the GII or the GI?

It's the longer receiver set and larger BCG that is used on the SR25 and DPMS LR-308 guns, which in simplified terms here means "G1" in the way you're framing it.

These terms are misnomers, other than the product line known as G2.

The Aero M5 is supposed to be compatible with many DPMS LR-308 parts, including the BCG, but I would get as much from them as possible to ensure correct fit and function.  You can use ArmaLite Inc. and DPMS barrels, with corresponding BCG's.  There are several other AR10 barrel sources as well, and many here have thrown them together, with mixed results in terms of function and form.

What LRRP said!

The M5 is a decent receiver set to work with. The quality is GTG. Just try not to cut corners, if you do a build. I'd suggest their BCG and LPK (over priced but gives you the correct pivot and take down pins) and a steel gas block. Also go with a good quality barrel with a MLGS or RLGS. And make sure you measure your gas port to ensure its the correct size. That's one of the many things LRRP has thought me.Very knowledgeable man.