It doesn't get you all Saturday 3pm games as these can be shown on TV in the US.
I believe it gets you all the games which are not televised in the US. This somewhat mirrors UK TV listings, but not entirely.
You can see which games were streamed online this week here...
It's not $50 for all the games of all the teams. To watch all the games of all the teams you have to have a TV subscription. The $50 NBC Sports Gold Pass gives you access to all the games which are not shown on their main TV channels, but it doesn't give you access to those which are. So with...