v2 users on .28 cant use replay gain at all LOL, we weit for fix from sansa support team, you can report that bug to, if they no noticed that v1 have a bug to, but at least you cqan use the function, and v2 users cant.
You can revert back to the x.x.26 firmware until they come out with a fix. The replay gain should work fine with that version. The latest one mainly has a video bug fix. See the edited first post in the current firmware thread for more info.