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No - G-Sync requires an Nvidia GPU and it requires specific models of Nvidia GPUs at that.

The only iMac currently with an Nvidia GPU is the high end 21" and the non-retina 27". Both are fantastic machines however G-Sync isn't supported on the mobile variants of the 700 series GPUs.

For a full list of G-Sync enabled GPUs, see this list:
https://en.wikipedia.org/wiki/G-Sync#List_of_G-Sync_enabled_GPUs

With that said, I really wouldn't worry too much about the lack of G-Sync support. Even if your GPU supports it, the fact is that very few monitors right now support it either.
 
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