A gel cell is just a battery , rechargeable or otherwise , that uses a gelled elctrolyte instead of a liquid one . I got that type in case of leakage , but any rechargeable will do . Mine came from Sportsman's Guide , but you can get them from the various feeder manufacturers and other places such as Cabela's and Bas Proshops .

Here's just one example:

Rechargeable battery

I also have one of those , it's a lead/acid type common to feeders , and that's a good price . They also carry the solar cell , and even a wall socket type charger for times when the sun just won't cooperate .