Does an external GPUs work with mid 2010 MacBook Pro with a mini display port , if I have a thunderbolt to mini display cable....?

No, external GPU is not supported on that Mac. No adapter can fix that.

ThunderBolt to Mini DisplayPort only works FROM ThunderBolt TO Mini DisplayPort. You cannot create a ThunderBolt on a computer that did not come at least ThunderBolt-capable. On a Mac, the computer had to come from the factory with a ThunderBolt port.