Knitting socks on CN, Somehow made a mistake and am not knitting in garter stitch after heel was knit, help!?

Best suggestion I have is for you to rip back to where the garter stitch starts, and purl instead of knit. When you knit in the round, you are knitting in straight stitch, so I think your garter stitch is really reverse stockinette. Check the other side, is it smooth? If so, the best thing to do is to rip back to where that starts, and purl around instead of knit around.