That GMX monolithic bullet (which is long for its weight) will not stabilize properly in your rifle, due to the twist rate BUT that is good news for your hunting and practice budget! The slower twist rate will allow you to use Nosler Partitions and other lead-core bullets designed for use in the 6.5x55, 6.5x57, etc.
If you want a harder bullet, the 139gr Hornady Interlock, which was designed to expand in medium game when fired from the fast-twist barrel of a Winchester Model 70 .264 Winchester Magnum rifle, may be the best option. That bullet has been known to pencil through the chest of a red deer spiker when shot from a 6.5x55 with the original 1 in 200mm twist but could be a very good option for shoulder shots with your rifle. Also, your rifling will last longer with that slower, and to my mind, more sensible, twist rate.
Rotational velocity is a function of muzzle velocity and twist rate.. It stays virtually the same until the projectile loses stability at maximum range. The greater the rotational velocity, the greater the friction generated by contact with the air, which is one reason why long range target shooters always use the slowest practicable twist rate in their barrels. Divide the number twelve (12) by the twist rate in inches for your barrel OR if using metres per second, divide a metre by the metric rifling figure. You can, by doing this, extrapolate design rotational velocities in fps OR m/s for various projectiles, not necessarily to Ballistic laboratory standards but well enough to select suitable bullets to test in your rifle. I suspect that RWS could probably make suitable recommendations in regard to usage of their lead-core bullets in your rifle.
Have a good New Year!