My first thought is to ask: how long have you been on Gliclazide?
Also, have you been on any other sulfonylureas in the past 20 years (particularly glibenclamide)?
I ask because sulfonylureas work my stimulating your pancreas' beta cells to produce more insulin. While gliclazide is considered to be one of the safer ones, in chronic usage (10+ years) there is evidence to suggest that (some) sulfonylureas can cause cell apoptosis.
In layman's terms, if you've been using them for 20 years, it's possible that your pancreas is losing its ability to produce insulin. I don't want to scare you, but it is a possibility, especially if you're not responding to the increased dosage.