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That certainly looks like a George Gray overstrike. The size of the letters, larger than the understruck mark, is typical. At 1792 the Bateman mark, as you have mentioned, would be PB over AB for Peter & Ann Bateman but, although there appears to be the top of a "P" there is not enough of the mark to be 100% certain and there are other bits below the GG which don't seem to match the necessary Bateman mark. However I can't think of another mark which it might be....
If the Bateman mark is upside down then the top letter on the left (in the picture as it is shown) could be partially visible B and the bottom right letter partially visible P so it would match with PB over AB.