I wrote:
A quick change like below might work if `gp' is found in your PATH. In the next release, passing `GP=gp' (or your command name for gp) will work.
However, expect some failures. I'm fixing the easier ones now, but some will remain in the next release (so no need to post those results now). In particular, there are some tests that test specific automake and low-level behaviour which will not work with gp. I'll have to consider how to deal with those tests, probably either skip them if gp is used, or disable gp for them. (But there are also some actual gp bugs in very obscure cases.)
A new GP release which fixes some of the failures is now at http://fjf.gnu.de/misc/gp.tar.bz2
Frank