I guess that AF has an interline agreement with AA, so you should be able to check your bags all the way through in Rennes. Just show the onward reservation, no BP needed (you might however get your BP for AA online, AF in RNS will almost certainly not be able to print an AA BP). You might need to insist, however, as at French domestic airports agents are not always aware of the policies. The morning flight RNS-CDG is usually on-time, so even if you could not check your bags through, you'll have enough time at CDG to do it.

Transfer 2G-2A is only landside with or without baggage. The only airside bus from 2G is to terminal 2E. Shuttle bus leaves you at terminal 2C from where you walk about 200m inside the terminal building to the A-extension, where you'll find the AA counters. You'll pass then passport control and security check at terminal 2A (I think it's now new, between terminals 2A and 2C). If you can't check your bags through, you'll have to get them at 2G and take the bus with your bags.