I have trawled through the latest info from the Gulfstream current aircraft pubs, and cannot find any reference to 60 minute ETOP limit. If it were to be fact, what would happen to Gulfstream sales? 2 years ago, Gulfstream publishes an OMS on ETOP operations. You can find it at
https://www.mygulfstream.com/portal/...20Rev%2000.pdf.
I can only think it must relate to Part 135 operation. AFAIAA there is no ETOP restriction on part 91 operations, just common sense!