A GameStop location on 6th Avenue on March 23, 2021 in New York. Check out the companies making the biggest moves in premarket trading: GameStop — Shares plummeted nearly 21% in premarket trading. The company announced Wednesday the ousting of chief executive Matthew Furlong and said Ryan Cohen would take over as executive chairman. Wynn Resorts , Las Vegas Sands — The casino operators both shed about 2% following a downgrade by Jeffries to hold from buy. The Wall Street firm said Macao's recovery is already priced into the stocks. Signet Jewelers — Shares tumbled nearly 11% after the jeweler…