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 stopped. In the past seven months there has been only a single one-line change to the gPXE tree, compared to well over 350 commits to the iPXE tree in the same time period.
In summary: if you are currently using gPXE then you should upgrade to iPXE as soon as is reasonably practical.