gnu: Add gmt.

[Giovanni Biscuolo]

Hello Ricardo,

checking commits done recently [1] I see you pushed ac86174e22 but I
cannot find the related patch sent to guix-patches: do I have missed the
relevant messages?

Thanks! Gio'

[1] I use git log to have a quick overview of latest changes and additions


P.S.. Generic Mapping Tools is a great package, kudos!

-- 
Giovanni Biscuolo

Xelera IT Infrastructures

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