No, a successful grapple attempt does not count as a hit.
Although a grapple attempt is called a "special melee attack," it uses a contested ability check (a contest) instead of an attack roll and, as such, does not result in either a hit or a miss (which are terms exclusively associated with attack rolls).
This ruling is confirmed by Jeremy Crawford in the Sage Advice Compendium:
Does a grapple or a shove trigger the Tempest cleric’s Wrath of the Storm or a Battle Master’s Riposte? The answer to both questions is no. The grappling and shoving options (PH, 195) don’t result in a hit or a miss.
When you make a Strength (Athletics) check to grapple or shove someone, are you making an attack roll? Again, the answer is no. That check is an ability check, so game effects tied to attack rolls don’t apply to it.
If it isn't a "hit," what is it?
The correct term for a successful grapple is merely a "success," according to the following rules of ability checks from the SRD (emphasis mine).
To make an ability check, roll a d20 and add the relevant ability modifier.... If the total equals or exceeds the DC, the ability check is a success—the creature overcomes the challenge at hand.... Sometimes one character's or monster's efforts are directly opposed to another's.... In situations like these, the outcome is determined by a special form of ability check, called a contest.... Both participants in a contest make ability checks appropriate to their efforts.... The participant with the higher check total wins the contest. That character or monster either succeeds at the action or prevents the other one from succeeding.
If it's a kind of attack, isn't the roll an attack roll by definition?
For clarification on the intent behind the written rule, see this response by Jeremy Crawford to a similar question on Twitter.
Does the [Sage Advice] on ability checks contradict this? It states that shove [and] grapple are not attacks? There's no contradiction. They are unusual attacks that lack attack rolls.
Divine Smite says (emphasis mine):
Starting at 2nd level, when you hit a creature with a melee weapon attack, you can expend one spell slot to deal radiant damage to the target, in addition to the weapon's damage.
This wording is a little ambiguous as to whether Smite is part of the attack damage or a damage modifier. Damage modifiers aren't doubled. However, Sage Advice says that Smite is part of the attack damage and crittable since it does not require a save or its own attack roll.
The rest looks correct so you'd have:
Normal - 1d12 Slashing
- add Brute (MM p346) - 2d12 Slashing
- add Orcish Fury (Xanathar's p76) - 3d12 Slashing
- add Maneuvering Attack (PHB p74) - 3d12+1d8 Slashing
- add Divine Smite (PHB p85) - 3d12+1d8 Slashing + 3d8 Radiant
- add Crit, and Savage Attack (PHB p41) - 7d12+2d8 Slashing + 6d8 Radiant
- add strength bonus and Great Weapon Master (PHB p167) - 7d12+2d8+15 Slashing + 6d8 Radiant
Great Weapon Fighting (PHB p72) allows rerolls for an initial 1 or 2 on the d12 dice. The Radiant Damage and Superiority Die is in addition to weapon damage thus not covered by the GWF feature. (Sage Advice)
Notes:
- It's important to track the damage types due to possible damage resistance/immunity/reduction
- Brute (MM p.346) and Orcish Fury (Xanathar's p.76) are not core Half-orc feats; not all DM's will allow them.
Best Answer
Yes, a hit that deals 0 damage for any reason is still a hit.
This is because the attack roll dictates whether it is a hit or not, not the damage roll (see Player's Handbook p. 194 in the section on Attack Rolls):
The attack roll occurs prior to rolling damage and dictates whether you hit. That is, by the time you roll for damage, the attack roll will already have resulted in either a hit or a miss. If it resulted in a hit, it still counts as a hit even if the resulting damage ends up being 0 when it is calculated later due to damage resistance, damage immunity, damage threshold (see the Dungeon Master's Guide for this obscure property), or what have you.
Note that many features that are reactive to attacks use different wording. A feature that triggers "when you are hit/when you hit" and a feature that triggers "when you are damaged/when you deal damage" are not quite the same thing. The "hit" kind of feature would trigger on a hit even if the damage is 0, whereas the "damage" kind would trigger only if the damage was 1 or greater. Therefore, be careful of the wording when evaluating such a feature.