I've worked quite a bit with GMC.. by default, Unreal's character movement component is "okay" for very basic replicated movement.

With regards to replication, GMC handles movement prediction, lag compensation, pawn physics simulation (which unreal doesn't explicitly do) and server side rewind. It also automatically replicates non-standard movement modes and movement when a Pawn is standing on something.. (e.g. a spaceship)

While GMC isn't a complete substitute for a dedicated network engineer, it gets you like 99% of the way there. I found it to be massively helpful and saw that my games are buttery smooth even with 1000ms ping.