From my rather limited experience, Gimp and Photoshop are broadly similar in terms of functionality; Gimp may involve more effort to get stuff done in certain circumstances, e.g. its selection tools can sometimes be of marginal usefulness, but it works well enough and people who are far more creative than I am seem happy enough with it!
You don't really need to know much (or indeed anything) about programming to use the stuff in question. I mostly use it to automate tedious tasks though it's questionable whether I really save myself any effort in the long run! Blender does use quite a lot of Python scripting but it seems most people pick it up easily enough if they need to; but in my experience, the need to do so rarely comes up.
Norbyte's tools are largely (if not entirely) reverse-engineered. In the case of BG3, modding is not yet officially supported and won't be until the actual release happens; and as such I probably shouldn't really be encouraging it, but on the other hand there's no prohibition of modding it either. Just that they won't (yet) offer help to modders nor of games that misbehave due to being modded.
There is no official certificate! If you wish to use the programs and indeed publish mods, just go ahead and do it. The only rules are to be aware of stuff like copyrights and credits. In terms of professional or vocational qualifications... I've no idea. I worked for many years as a programmer and sysadmin and have no pieces of paper saying I can do either. Well, other than whatever it was I got from college years ago, and that doesn't even have my correct name on it.
As for positions, it can't hurt to ask. I can give no further advice on that front as I am not an employee but it might be interesting to see what they say. All I can say is to repeat my own experience of looking to work in a particular career which is to keep trying and not to be put off by any knock-backs and non-responses: it happens all the time and is just the way of things. Also remember that applying for any position is a two-way process, it's not just about whether they want you to work for them but if you like the look of a prospective employer: don't feel that you have to accept the first thing that comes along (I mean not unless you really want to).
