After a bit of concern, and working through various things, I did install Gpredict 2.2 on a Mac Mini, Yosemite (10.10.5). The key was reinstalling the right MacPorts version for Yosemite and then sudo install port gpredict. Now I am gonna go install on a 2011 MacBook Pro with Capitan (10.11.6).
I had thought that I had the right MacPorts version - but maybe not. I got a bunch of errors on my earlier attempts, errors for dependencies.
Oh well it worked and seems to run great. Now to use it.