Quoting the FAQ: iPXE is a superset of gPXE: any feature present in gPXE is also present in iPXE. Any gPXE user can seamlessly upgrade to iPXE as a drop-in replacement.
iPXE is developed by the people who originally developed gPXE (which evolved from Etherboot). Unfortunately, the gpxe.org and etherboot.org domains are owned by an individual who wishes to exercise a high degree of control over the project and the codebase, so in April 2010 the decision was taken to create a new project named iPXE, using the existing codebase as a starting point. Since the two projects diverged, development on gPXE has stalled. In the past five months there has been only a single one-line change to the gPXE tree, compared to well over 200 commits to the iPXE tree in the same time period. In summary: if you are currently using gPXE then you should consider upgrading to iPXE as soon as is reasonably practical.
However, the orginal etherboot/gpxe wiki provides most of the information to deal with gpxe/ipxe.