PEX takes its name because it is a form of polyethylene with cross-links, hence the X in PEX. The plumbing material has become en vogue because it doesn’t require glue and it’s less likely to burst if it freezes.
Find out how to plumb with it.
GPF stands for gallons per flush, which means how much water you’re using and how it will flush. When toilets started going to 1.6 gallons per flush it meant a lighter flush but that problem has since been solved.