None of this is hard if you have clue what you're doing. Rebuild the Pg gem against the right libpq by fixing your PATH so it finds the right pg_config, set host=/tmp, or set host=localhost. Any of the three will work. Unfortunately most of these users seem to struggle with that, and their approach to "it didn't work" appears to be "find another tool/tutorial and try that instead".
So we need an "official" tutorial? But which distribution would we point people to? :)
The postgres.app documentation its self doesn't look quite right when it comes to Ruby, actually. For Ruby/Rails it says the user should use "gem install pg" but it doesn't tell them to set the PATH first, so they'll get whatever random pg_config is on the PATH first, often Apple's elderly Pg with its different socket directory path, etc. Sure, they can get around that just by setting host: localhost, but it'd be nice to see that improved so it tells them how to build their Pg gem against the correct libpq. Or, better, has Postgres.app automatically install it for them when they install it.
Hmm, but where to install it? People using rvm or bundler will have their gems tucked away in a variety of places.