How can i know if my mid2010, core i7, iMac has a graphics card (ATI Radeon HD 5750 1024 MB) that uses a GPU.
If so, how can i enable it for Premiere cc2017?
It means, that although it is a GPU (as in: Graphics Processing Unit), it is not advanced enough to give you any substantial performance increase in Premiere 12.0 - in terms of processing the video. The CPU will do all the heavy lifting.
i think that gpu must support metal framework now to properly accelerate. Oherwise even internal intel igpu is probably faster than your card. I had similar card and it was not useful that in mojave. Ended up exchanging by rx 560