To list all the differences would lead too far here (to start with the different architecture of the used controllers) - what in particular do you want clarified?
If you want to use GPS with Gen 3 the go-to option at the moment is a FeatherWing Doubler/Trippler and any GPS FeatherWing you'd like.
The "legacy" thing isn't the best of wording used there, but a similar question has been asked before and this is Particle's response
(plus another post a bit further down the same thread)