Check on Fuelly for some good overall numbers to gauge an idea. My ST use to get circa 55-58MPG, and i know the GT's were more efficient than those with most achieving higher 60's/early 70's if ridden sensibly!
I bought one of the last F800GT SE in 2020 as my R1200RT was getting too heavy. The F800GT is around 50kg lighter.
I have regularly returned mid 70s mpg and on one occasion, riding gently to a club meet totalling 120 miles it returned 97 mpg!!
15 litre tank so usually get over 200 miles to tank full.
Same engine as my F700 but higher state of tune .I get 230 miles per tank so recon 200 is bob on the money. Does the 800GT have the quirky ( I only show you the last half of the tank) that the F650/800/700 GS’s have?