The GC supplier I imagine you may have gotten yours from has an adapter for KR8/901 to GC now, altho it's out of stock atm. (Will add, myself and another person I know of ordered it right before they ran out, and
apparently received the wrong ones, which I would guess was just a fluke others don't need to worry about. Didn't work with V4L autos or manuals of any length. Know of a couple others using it successfully, tho not sure of the batt brand in both cases.)
But if you mean that other (
non-V4L, non-XL) auto KR8s you have
do work with GCs without an adapter, dunno the details on that. Tho I do recall reading that one or the other of auto or manual KR8s work without an adapter, which was confusing because it doesn't seem to match what all users end up reporting.
The poster 'MikeDeason' recently commented over in that GC territory that he specifically liked the adapter referred to above with his V4L batts; and I
think he uses or also uses autos, but not sure offhand or sure what length. (He has lots of posts in this V4L subforum tho.) Maybe you could try catching him on that other thread 'over there' too. That supplier may not know for sure either what the diff would be, as many seem not to stay abreast of product inter-compatibility. FYI, my V4L batts are all from 11/09 to about 01/10, and V4L's batts were tweaked after that, tho I wouldn't
expect that to be related here. Not sure when Mike bought his but think some have been recent ones.
Then there are always the workarounds like perforating a small hole in the 'G' of the carto's 'GreenCig' logo, or using the staple mod. (Fun fact lol, I also have an adapter received in an order from China that works, altho it didn't claim it would and I have no idea why it does! Real helpful that, eh?

)