According to pcpartpicker there are zero 25inch 1440p monitors with a refresh rate above 120hz.
https://pcpartpicker.com/products/monitor/#F=635000000&r=256001440&D=120000,240000
You'd need to go with either a 24 or a 27.
Any particular reason you need gsync only? Most freesync monitors, even if not on the compatibility list, are gsync compatible. And it actually works fairly well, most of the time. It'll probably save ya some cash too.
There's a ton of monitors, both 24 and 27 inch to choose from. I have a dell s2719dgf, it's a 27, 155hz. Freesync, but gsync works great on it. It has favorable reviews, both here and from consumers. As well as myself. It should fit well into your budget also. It is a tn panel though, but to...