Welcome! It's called a countermark, or counterstamp. Here's an article on some of the more interesting ones: http://www.numsoc.net/countermarks.html They were applied to coins for many reasons. Theoretically, it may be possible to determine who placed this countermark, and why, but with a simple G, it may be difficult.
Thank you very much. On the reverse (just above the Welsh feathers) there is another, much smaller mark which look like initials (ERS possibly). Will post a pic if anyone is interested
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