if its just thickened as opposed to setting up in the pot, mix some brand new sealer or clear into it....you can't thin it a lot and you shouldn't do it more than once but it does help use up the last gloopier bits.
Add it a bit at a time....never more than about 25% clear to 75% white, and mix mix mix....way more than you would to stir the gel. then let it settle overnight before using it.
I never buy bigger than 4ml pots now for this very reason.....and I often use the last bit for mixing for custom colours.
Theresa is right though....once gel has started to set up in the pot - there is litel that you can do with that.
HTH x