First off I hope I don't come across as extra since there's a few deets involved but before anyone give a right answer they'd need to know where exactly in the check-in area you'll be starting from.

BKK Airport has approximately 20 check-in rows (or islands) – A, B, C... all the way to W.

Regardless of which row, or international departures / fast track zone you use, once you cleared security and exit control, you'll be turning right then walk straight down to the 'E-F-G' intersection, and then turning right again for G concourse.
FWIW, there is only one travellator along the way and another one once you get to Gate G1. You will have to spend quite a bit of time on foot.

If you are using Rows A-E, then I'd expect you to use the Fast Track near Row A, which would return around a 15-20 minute walk (850m) to Concourse G. Even for those in good health, it's quite a bit of a walk.

For G-N, it's slightly less at around 15 minutes (600m).

Finally, for P, Q, R, S and all the way to W – it's going to be a 10 minute walk (max) (300m).

... and finally, which gate you get will add anywhere from zero to 350 metres. G concourse has 5 gates.