to my knowledge PCBExpress will not generate Gerber files they want to lock you in to buy the PCBs
If you want Gerber files generated PCBExpress will do it for you for a fee but it is not cheap.
I have used many CAD programs over the course of my career (PADs, Mentor, PCAD, Orcad, etc) now
I am using Diptrace, it is affordable and has a large library of components. I think they also have a free
very limited version.
73, Yvon NU6I