The gall bladder removal was 12 months ago - could there be a lasting effect of its removal on the gamma gt?
Not from the removal itself, no. Any effect the surgery alone had would have resolved by now.
Is your own doctor investigating this further for you?
There are a lot of causes of a raised gamma GT including medications, alcohol, pancreas problems, kidney problems to name but a few.
However a raised gamma GT in isolation can't tell you which, if any, problem you may have. There are lots of other liver enzymes that should be checked along with the gamma GT (so that the overall function of the liver is known) and the same blood test can check the kidney function too.
This is a good website that discusses liver function tests: http://www.patient.co.uk/health/liver-function-tests
I would ask your GP if your blood tests included the rest of your liver and kidney function and if not, request that this be checked too.
All the best.