Let's hope I'll never have to refer back to this page.
What I did
irb(main):001:0> require 'google-drive'
NameError: uninitialized constant Middleman::Sitemap::Store::Pathname
What 'fixed' it
gem uninstall middleman-core
Which was at version 4.2.1, followed by:
gem install middleman-core -v 3.4.1
This fixed the immediate issue, but was followed by new problems.
What I should have done instead
irb(main):001:0> require 'google_drive'
There's a big difference between the gems