`yakbak` is about service virtualization, to help you remove 3-rd party dependencies for your tests, and replace them with recorded responses.

`diffy` is a reverse proxy which multiplex requests to multiple endpoints, compare differences and detect regressions between responses.

`gor` is about interception your production traffic (it is not a proxy, more like a network analyzer), continuously, and replaying it on demand to your test environments (you can modify and filter this traffic using various ways).

Hope it helps.

PS. I'm Gor author.