I have the press-in grates.
They are made of pot-metal. As Joules5 says, and as I recall, each are held in place by 8 studs that mate to 8 plastic studs.
My recommendation is to pry slowly around the grate at the stud points using custom "fork" tool that fits around the "fins" and lifts along the edge. I would not recommend lifting on just the "fins".
B.G.
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