The 34-year-old forward left the Eagles earlier this summer and has now been reunited with Sam Baldock, his strike partner during their time at MK Dons.
"He's played at this level before and was a regular goalscorer at about a one-in-four ratio," boss Steve Cotterill told the club's official website.
"He's already enjoyed a terrific partnership with Sam Baldock at MK Dons, so that's a major plus. I also think he'd work just as well with Jay Emmanuel-Thomas.
"I actually bumped into him on holiday and I'd seen him in the gym and sussed him out already. I threw out a bit of bait and he liked what he heard.
"Aaron is a top lad and a top professional. He's certainly a physical presence up top and that's what we need in the squad."