Can an iMac (21.5-inch, Late 2012) can be connected to an external gpu?

That 2012 iMac does not support external GPUs.

It has Thunderbolt 1 ports that run at a maximum speed of 10 Gbps. The 40 Gbps Thunderbolt 3 ports mentioned in the Apple Support article did not appear until many years later.

I have never seen it done so I can't speak too much on that but here is an article that may be of some use: Use an external graphics processor with your Mac - Apple Support

Thanks for the clarification