If I use Metang's "Metal Maker" Ability while my opponent has Gengar ex with "Gnawing Curse" in play, do I put 2 damage counters for each Energy card attached from "Metal Maker"?
No, Metal Maker does not attach Energy cards from your hand so Gnawing Curse does not apply.
Source: Temporal Forces FAQ; TPCi Rules Team (2024-03-21)

Relevant cards

Note: All applicable cards may not be listed below.
SV5_114
SV5_114
SV5_104
SV5_104