MarsSeed [1] filed a deletion request for gmt4 [2]: Proposal to delete this legacy gmt v4 pkg from 2018.
Also upstream website says: 403 The project you're trying to access has been archived. Extra repo carries the latest gmt. [1] https://aur.archlinux.org/account/MarsSeed/ [2] https://aur.archlinux.org/pkgbase/gmt4/
