So me and my wh40k loving pals decided to have a brainstorm on skype about the abnormal vertical velocity Gabriel is able to produce in his terminator plate, and these are our resulting theories, all based on either the lexicanum, speculation or pure logic.
THEORY I - The God-splitter theory - The God-splitter is an artifact containing a shard of the Wailing Doom, a non-mortal weapon capable of wounding demons and has a large psychic power contained within it. If we assume that the weapon was constructed witht he purpose of combating powerful demons, then its wielder would require aid beyond a good Hammer. It has been scientificly proven that striking ones face is the most effective way of elimination, and as powerful demons tend to be quite large and tall, it seems only logical that the hammer itself would aid its wielder with accelerating him in the direction of his choosing, be it vertical or horizontal.
THEORY II - The Physics theory - Terminator armour comes with electrically-motivated fibre bundles that allow the wearer to move in this exceptionaly heavy suit of protection. It has a static amount of force it can produce, BUT the weight of terminator armour is not static. If the planet on which the action of DoW 3 is situated is exceptionaly small and/or compries of lower density matter then the gravitational pull would also be weaker, making the suit's weight subjectively smaller allowing its fibre bundles to do more than on standard earth mass body.
THEORY III - The Armour theory - As the Blood Ravens are known for their "lust" for aquiring relics and artifacts, Gabriel's armour might be just that. A relic long lost constructed with the purpose of mobility in mind. Having either more or higher tech fibre bundles, or supported by different tech like pneumatic servos and a larger power supply might explain his acrobatic prowess.
THEORY IV - The Warp Did It theory - Either gabriel is a chaos follower or has immense psychic potential, the planet is a home to large amounts of warp energy or some other sort of warp manipulation. If gabriel belived himself to be able to summer-sault through the air enough and there was either enough warp energy or he's a powerful psycher, then that would be the case. Quite an unlikely theory but never the less a plausible one.
We have come to the conclusion that it is most likely a combination of the first 3 theories in some proportions. If you have any other ideas for the cause of Gabriel's leg strenght you are welcome to post them here, and we can have a discussion about it.