There are two distinct items with the same user facing name: "Iridium Plate." This is a bug. One of them is indeed a vanilla IC2 iridium plate; you can make it by the process [Iridium Ore] -> Compressor => [Iridium Ingot] -> Plate Bending Machine => [Iridium Plate]. The other, however, is a much more elaborate GT recipe involving four of these vanilla Iridium Plates and other ingredients in a rolling machine, resulting in a GT item called an [Iridium Alloy Ingot]. This is a rewrite of a suppressed IC2 crafting recipe, the one that takes iridium *ore* and other ingredients, and produces a vanilla iridium plate. In GT, you then take the [Iridium Alloy Ingot] and process it in an implosion compressor, to make a new item that is *also* called an [Iridium Plate]. This one is completely distinct from the vanilla IC2 item of the same name; the two cannot be interchanged in any recipe.
The right way to resolve this, as far as I can tell, is simple: GT should use a different name for the result of running an [Iridium Alloy Ingot] through the implosion compressor. I suggest "Iridium Alloy Plate" would be ideal.
This approach has the happy property of not having to change *anything* except the user-facing names. All of the block and item IDs etc can remain unchanged.