It makes your pancreas make more insulin. And insulin has a secondary function of helping you store body fat and inhibiting losing it. So yes weight gain is common but not automatic.
Why are you going onto it? What else have you tried if you are reluctant? What style of eating do you follow?
I have personal experience of losing 8 stone while taking max dose of this drug (Gliclazide) I am still on it, and my weight has been static for 8 years now with a BMI varying between 21 and 22. I did it without exercise or calorie restrictions. didn't even count carbs. I still don't.
If you have high insulin resistence, then Glic will increase insulin, and this forces the body to store fat rather than burn it off ergo weight gain. Most T2D have Insulin resistance, so for many this drug does not seem to work.
I used a low carb diet to reduce my insulin resistance, and when I started the Glic was not working, and now it works fine on minimum dose and my sugars are in control. It took a couple of months of LCHF to clear the IR, so it is not something you can acheive instantly.
So if the bath is full, glic will cause problems with overflow. All it does is increase insulin levels, and IR prevents that being properly used.
If you test before and after meals that will guide you what amount you can cope with. A rise of less than 2mmol at 2 hrs post meal is a goal to aim for.