Mac Pro 2013 eGPU

Although it is not officially supported by Apple there is a workaround published on egpu.io.

PurgeWrangler https://egpu.io/forums/mac-setup/script-enable-egpu-on-tb1-2-macs-on-macos-10-13-4/

It is still rather experimental and you should proceed at your own risk at this stage, but there are many who use it and positive reviews about it.

Terminal knowledge is required.

eGPU support is only on new Macs & doesn't stretch back to the current Mac Pro.

eGPUs are supported on MacBook Pro notebooks released in 2016 and later1, iMac computers introduced in 2017 and later, and iMac Pro. Your Mac must also have macOS High Sierra 10.13.4 installed.

Ref: Apple KB - Use an external graphics processor with your Mac