To add some approaches not covered in answer: A recent one without the plugin but all the latest core features, and either an older GIMP with plugin, or the GIMP on VM route. That said, its probably easiest (and a huge hassle) to just install two versions of GIMP, like in the 4th option. Use that for the Enhance filters (if you work exclusively in the Virtual Machine you might of yourse skip the macOS part of installing GIMP) Install the official or any other binary for GIMP as usual on macOS, then install a virtual machine and a Linux in that, with GIMP and the resynthesizer plugin ready-made to play well from a package repository.

Use the help of MacPorts to build everything GIMP from scratch/source including all build-dependencies and then go to GIMP Resynthesizer Plugin Suite and build that as well from source. Currently at McGimp 2.10.8 Standard Edition (2019). Does not include the resynthesizer plugin itself but has recent builds against which building the plugin itself is a bit easier. The 'some plugins already included approach: download a binary from Partha, Photography and Related Musings. On Sourceforge the version recommended by Resynthesizer Gimp on OSX Gimp-2.8.18p1-Sierra.dmg ) (Original website no longer active as of May 2019. Currently the latest version is 2.8.12 built for Sierra.

Sadly development seems to have stopped about a year ago. The batteries included approach, as recommended b resynthesizer: download a build from GIMP on OS X. 'Easy' as in 'still a pain', just like the other, third and fourth option. There are currently two relatively easy ways to get to this plugin.

Get gimp for mac