Whoa. That would easily be the most powerful melee feat in the game because of its synergistic qualities with the rogue multiclass. Sneak Atrack + Great Weapon Master + Polearm Master (specifically it's off-turn qualities) is a combo that the designers went out of their way to prevent for very good reason.
Yes, I realize this feat doesn't allow Polearm Master, but Sneak Attack + Great Weapon Master + Sentinel isn't much better. Either way, you're probably getting two instances of sneak attack per round on top of your already top-tier melee damage. That's far more damage than any other fighting style can contribute. And if you pick up a second fighting style, Great Weapon Fighting's rerolls would work on sneak attack damage, as well.
I'm not sure about that. Remember you're still only dealing 1d8 damage two handed.
And as the rules currently sit I don't see a reason why you can't use great weapon master with a spear anyway as it is versatile.
Finally I see nothing in the rules to support that GWF rerolls would allow you to reroll sneak attack as they are extra damage and not weapon damage, nor would I allow it at my table anyway. Maybe I'm missing something here in the rules but as written I don't actually see a problem.
Not sure who came up with this feat. It's on the site here though...
Fighting Style (Fighter, Ranger, Paladin)
While wielding a spear, you may treat it as though it had the Finesse property. If you wield a spear two handed, you may treat it as though it had the Heavy and Reach properties.