How well does liquid antimatter fare against gold and platinum arena?
This is highly variable, depending on how many upgraded cards the version you're using has, and the general meta among Arena decks. You'll generally find yourself getting outrushed by smaller creatures, which the combo generally ends up failing against.
Right, so what is the general metagame in Gold and Platinum? Are there minimally upgraded decks that can grind for occasional upped cards there?
I can't speak for Gold, but Platinum has a very broad metagame. Expect to see a lot of decks using Shard of Sacrifice, Poison, denial in the form of Pests, Quicksand, Black Hole, Discord and Silence, Speedbows (most of which use Discord and Black Hole), healing (Vampires, Purify, Black Hole, Sanctuary, Shard of Gratitude, Feral Bonds, etc.), Ghostmares, and Fire. Liquid Antimatter fared somewhat averagely when Arena was first released, but now there's just no way it could handle it. I also find it extremely difficult at times to get a special spin with my current grinder due to losses breaking my consecutive chains of wins, so I have severe doubts about a mostly unupped deck managing a special spin without ridiculous amounts of luck.
Assuming that Gold is a watered down version of Plat you may have a better chance there, but I still have my doubts about winning upped cards being frequent enough to be considered "occasional" there as well.