This program is part of Netpbm. pamstretch-gen is a program which uses pamstretch and pamscale to smoothly scale up a Netpbm image by any factor; it's like a more general version of pamstretch (hence the name). pamstretch-gen uses the -dropedge option on pamstretch.
There are no command line options defined specifically for pamstretch-gen, but it recognizes the options common to all programs based on libnetpbm (See Common Options.)
The Netpbm global options -quiet and -plain did not exist on pamstretch-gen prior to version 10.86 (March 2019). -quiet did not work until version 10.91 (June 2020).