I would say the Momentum slam gets the +2 bonus to slammed distance and the great bear can follow up to the distance the slammed model was moved.
Gladiator does not give a benefit to momentum because momentum is not a power attack.
On second thought something came to mind, can the great bear move directly towards the target with “follow up” if the enemy model did not survive the attack from the heavy cannon? Because, how do you move towards a model thats no longer on the table?
As towards that, I’m actually not a huge fan of the interaction unless I’m gaining the extra follow-up movement out of activation. If I can manage that somehow, it is indeed a better ability than repo, but shooting, slamming, hoping it survives, and then following up can be a little tricky to use effectively all within the great bears activation if I also want to do melee work. It could be spicy in Boris potentially, but there’s a little bit of skornergy where you want the model to die to keep chaining boris’s feat, but you also don’t so you can follow up. It’s weird, repo can be much easier to use
You can have the follow up out of activation. Once with “Ancillary attack” from Volkova, once with the spell “Avenging force”, once with Ilari’s feat and once with the spell return fire. Theoretically the great bear can traverse extreme distance with follow up, sadly there are many “if’s” like if you hit the enemy with RAT5 and if the enemy survive your pow16 attack and if the enemy don’t get stuck on something after 0,5 inch after being slammed.