As long as the velocity you use is an actual velocity measurement from your specific setup, and you know with certainty the bullet is the #2155, all you have to do is plug those values and the appropriate atmospheric conditions into a ballistic calculator as @daniel.chisholm suggested. Using a generic "box" velocity value will only get you as close as the box velocity value is to your true velocity with that specific ammunition. It might be close, it might not.
Once you have reasonable predictions from a ballistic calculator, they should get you close enough at longer distances, although no ballistic calculator should be expected to be spot on to a single elevation turret click at every distance, on every different day. You can tweak the settings and record the values to improve your dope over time as you encounter various conditions, etc.
Hopefully someone else will reply with more up to date information, but when I fired GGG .308Win in 2015 at Bisley it was a Sierra #2155 loaded to 2900-ish fps (from memory; actual measurements from the batches being fired were posted at the time on notice boards).
So, unless they are using a different bullet, why not just use the predicted ballistics info from a #2155 Sierra at 2900fps m.v. (or actual measured m.v. from your rifle)?
My GGG is the GPX15 .308 175gr Match Ammo. Any help on the actual bullet design would be great! The data sheet suggests a HPBT BC of 0.496 and an out of the box Velocity of 810 m/sec +/- 7 m/sec.
The GGG 308 Match supplied under the GB NRA contract for use in UK 'Target Rifle' is loaded with off the shelf Sierra 155gn MKs p/n 2155 as Daniel Chisholm says in post 2, nominal MV 2,925 fps in a slightly 'tight' 30-inch barrel. Actual MVs will obviously depend on the barrel internal dimensions, length, chamber freebore and condition.
Use a 300 yard drop
Measure drop from 100 yard zero in inches on a target
do not adjust sights or scope for a 300 yard
Measure offset in inches
then mess with the velocity numbers to line up the drop in inches
That will get you a good idea of a trued velocity
Hearsay has it that this round also uses an off the shelf 175gn MatchKing. Litz's average G1 BC of 0.475 through his testing appears to confirm that being reasonably close to the claimed value. For ballistics programs Litz's measured average G7 BC of 0.243 provides a more accurate metric for 500m and longer distances.
Most factory 308 Win nominal MVs are measured in an industry 'standard' (internal) dimensions and chambered (the European CIP in GGG's case, this being a Lithuanian make) 24-inch barrel. Depending on your rifle's spec and condition, the quoted 810 m/s MV may be well out, and you really need the actual MV in your own case. However in the absence of a chronograph and the relatively short range of 500 metres, it'll be close enough to get you going, unless you intend to shoot in something that doesn't allow sighters.