This, or more specifically, moves* actions* – each time you match gems or cast a spell, it deducts ~1% * the base score (i.e. 250-350 depending on the battle) up to a max of 50 moves (after which you don’t lose any more points).
I say ~1% because of rounding. E.g., Shimrra’s score above is 14 points from the max of 1,820, which checks out as 4 moves * 3.5 (with the base score being 350 for the Paragon battle). igniteice’s score in the top post shows 24 points from the max, where (assuming everything else being maxed, which is a safe bet) the 7th 3.5 is rounded down from 24.5 (Banker’s or just downwards rounding).
I think Zuul does count, with it treated as spell damage (this also shows up on the MVP screen in PvP), but could have it wrong (e.g. I know it was stated Scorpius’ damage wouldn’t count when GW was introduced, but maybe the way his spell works has changed since then). I can’t remember what the deal was for Devour, but I know @blindnighto tested it to confirm one way or another.
Also, a note that
was changed in an old and hard-to-find weekly event post to 50 moves*:
and should ideally be updated in the Help Centre article, which still shows 30 (@Jeto @OminousGMan 
):
