I have copied some patches from Debian to have gpart running on my amd64 arch
Created attachment 210782 [details, diff] gpart-0.1h-r1.ebuild.patch The patch for the current ebuild
Created attachment 210784 [details, diff] gpart-0.1h-l64seek.patch The patch adds the support to amd64
Created attachment 210786 [details, diff] gpart-0.1h-openmode.patch This patch solves a compilation issue
Fixed. Bumped to -r2