Since recently gallery requires imagemagick instead of the previous default of netpbm. I have adapted the ebuild with a local use variable netpbm, so people can now make a choice between the two. As a bonus I also added some missing platform keywords. Reproducible: Always Steps to Reproduce:
Created attachment 14776 [details, diff] gallery ebuild with possibility of netpbm support
if both detected and installed, you can choose in setup-wizard which image manipulation tool you want to use.