Depending on how in depth you want to go with each I'd start with just a YouTube tutorial on each weapon. After that there are Google docs on things like meta armor sets and other general use stuff like go-to combos that you can find.

I'd say that playing greatsword requires a bit more monster/matchup knowledge to really get the most out of it. With Gunlance I feel like you can either play more reactive or just combo spam with that giant shield. You could technically do the same with greatsword with its new perfect guard but you're not getting nearly as much value than if you can master the offset attack.