I have herd a few things about the side effects of Gliclazide and one is weight gain.
If you google Gliclazide and weight gain, weight gain is not uncommon.
My own personal experience was that after being taken off Rosiglitazone, an awful drug which helped me gain a huge amount of weight I started to lose weight (no change in diet of exercise, I still have my food diaries). I had lost 4 stone in 4 months, without trying (been low calorie for ever) trouble was my BG was not good so put on Gliclazide at which point I stopped losing any weight and in fact gained 10 kgs.
Judging by the other replies it seems to go either way, although weight gain is one of the key facts noted in the NHS guidelines and is also noted as a side effect in the British National Formulary.
And NICE have this to say:
Cautions For all SULFONYLUREAS Can encourage weight gain (should be prescribed only if poor control and symptoms persist despite adequate attempts at dieting);
and:
Sulfonylureas are also associated with modest weight gain, probably due to increased plasma-insulin concentrations.
At least that last bit says why it might happen, rather than assume people eat more.