OK, first just to be sure, you apparently want to build gretl from git
yourself, or you don't need any of the build-dep stuff.

Next, I only have an older Mint version to look at, but there is a
system settings window called software package sources or something like
that (sorry, I'm re-translating from German). In that window, you just
have to tick a box (at the bottom for me) saying "activate source
package sources" or similar. Afterwards, you reload the sources via
apt-get update (or the GUI equivalent)

In fact, I think you do: the "source" repo contains all the -dev versions
of the packages we link against (GTKSourceView, for example). The command

apt-get build-dep gretl

is a quick way of installing all the things that a debian-based distro
needss to build the gretl .deb package

I think we're talking past each other and creating misunderstandings,
because there is no disagreement. Perhaps I should have written
"otherwise you wouldn't need" instead of "or you don't need".

In any case, the "deb" lines posted by Clive above look very wrong to
me, because AFAIK the download paths from sourceforge are not
Debian/Ubuntu/Mint package repos in any way.

And taking one step back, are you sure the deb-src things are really
needed? The dev packages are needed for the build, yes, but they do not
live in the deb-src area AFAIK. Given that the gretl sources are also
not taken from the Debian/Mint release --instead the current sources
from git are used-- I don't see how any source packages are needed.