For those who don't know yet (if there are such people here at all

).
Lore-wise, grav-guns are Crusade-era weapons that use target's own mass against itself, although it is only capable of affecting limited space, so they are less effective against large targets despite their mass. Apparently, AdMech managed to get their mechadendrites on intact STCs and kickstart mass-production of these weapons, mostly for Space Marines forces.
In tabletop, grav-gun is salvo-firing weapon that works best against infantry with heavy armour, but almost worthless against models with light armour. It doesn't work well against vehicles, but is capable of immobilizing them and slowly grinding them down. Grav-pistols, combi-gravs and heavy grav-canons also exist. Their primary weakness is short range - only grav-canon is capable of firing from bolter range, but no further.
Due to wonky wording of rules, this weapon ignores (by RAW - rules as written) invulnerability saves (something that usually represents protection from energy fields, daemonic essence, being a scrap heap on wheels that was defying common sense even before you made another hole in the hull etc.) of vechicles. This fact, combined with obscene amount of shots weapon is capable of dishing out (and availability of relatively survivable infantry platform for them), made them a weird weapon of choice against flyers as they can saturate almost anything with bucket of dice they vomit, and shortage of dedicated anti-air ensured that dice saturation is the most reliable and accessible way to fight elusive flyers for some armies. Almost like it was in early XXth...
This weapon system plays game of tradeoffs primarily with plasma weapons due to shared main purpose.
There are numerous concerns with this weapon when it comes to implementing into DoW2, especially on TM, but primary ones are lack of distinctive role and the fact that TM already saturated with gun options. Heavy grav-canon could be interesing option if one day you will get the idea that TM needs setup weapon though.
On presented visuals: I think I like the model, although since DoW setting isn't quite rife with recently rediscovered tech, it could be done to make it look more like a relic item... What I'm not sure about is the way firing is visualized. That may be just me, but I was picturing its effects to be more subtle.