Newbie question

You need to understand the processes to hazard a guess at your question. Highly simplified but I hope still accurate ....

Metformin works mostly by reducing glucose dumps from your liver when it thinks you might need them. Eg on waking or after not eating for a while/exercise. It doesn’t always get it right in type 2. It helps but isn’t usually a massive number changer. It’s designed to go alongside the lifestyle change you’re making.
Gliclazide makes your body produce more insulin. (Rather than injecting it for example) Forcing the body to accept and use insulin by saturating your blood with it til it eventually works. It makes much more direct effect on bgl than metformin does

Type 2, especially in the earlier years (decade or more typically) have way way too much insulin already but we aren’t responding to it properly. We’re trying to deal with blood glucose as a normal process, failing due to insulin resistance/massive carb overload so make more insulin thinking that’ll do the trick. For a time it does. It might have kept bgl normal for years but in the background insulin was really high in order to achieve this. Then that’s not enough either as the insulin resistance has got worse and it becomes a spiral resulting in ever increasing bgl and diagnosis and ever rising numbers, til in some people, eventually, the pancreas gives up “exhausted “ and then fails to produce enough/any. Many t2 also have visceral (belly) fat making the process inefficient.

So my thoughts would be when you stop gliclazide you might well see a bit of a jump. But the improvements you’ve made to diet (Low Carb I hope, or do you mean cutting calories and following traditional advice) , any weight loss, especially around the middle and more movement and exercise will all be helping improve your sensitivity to insulin and your own supply/use of insulin unhindered by visceral fat. Hopefully the net effect will be fairly neutral. And with time and continued good low carb eating reach a level you are happy with.

Can you confirm the name of the drug please?

It's worth following their suggestion and coming off all medication to see what your numbers are. You can always go back on the metformin if they're unhappy with the results.