Welcome!
There’s always interest in documenting GEDCOM interchange idiosyncrasies. The Wikipage is easy to expand. And sooner or later, someone else will find it useful – even if they just use Gramps as a cleanup tool while transferring the data somewhere else.
Gramps sometimes complains when the GEDCOM is non-standard and then makes a pretty good guess about what was meant. And when the importer complains, it generally preserves the original GEDCOM line so a post-process can be applied.
You can use the Isotammi SuperTool add-on to do mass edits.
We have an example Discourse thread about using it. But Patrice (@PLegoux ) has done a lot more exploration and published lesson on the French Geneanet site.