Unfortunately gsync adds a massive premium to the monitor and you won't find anything at that size and resolution which is in your budget (looking at OCUK and I think you'd still struggle to find a physically smaller and lower resolution with it).
You can still benefit from the higher refresh rate on freesync monitors - you just won't get freesync and you'll have much larger choice - essentially you'll just be upgrading resolution and potentially refresh rates from where you are now.
Quick look through these will give you an idea of whats available
https://www.overclockers.co.uk/monitors/by-type/144hz-?sPage=1&sSort=3# or even
https://www.overclockers.co.uk/monitors/by-size/27-68.58cm and unfortunately there isn't anything at that price point which meets or really comes close to your requirements.
It's a case of spending more or reducing your requirements and probably dropping both resolution and physical size to get something in budget - which isn't sounding like a great upgrade given what you already have