As the author, a benefit it offers you is protection against someone selling your work or a derivative of it, or redistributing it with added restrictions.
From a user's perspective, the main downside is that using GPL software as a dependency makes the user's project a derivative work, which requires them to license their own project with the GPL (or from a selection of other compatible copyleft licenses).
In that sense it's considered a "viral license," i.e. everything downstream of the GPL software needs to adopt GPL. This is less of a problem for an application, but is more troublesome for a library (with many dependents of its own, with dependents of their own, and so on). It also means that if you use it in commercial software, you essentially need to open-source all of it.
So in your case, just using Marcel as a shell with GPL doesn't put any licensing obligations on the user, but using it as a library does.
I hope that helps! Someone else feel free to chime in if I misrepresented any of that.