Open-source art was an idea I presented, and since had gained support by the moderators since that one time I wanted an XCF of an art piece I saw on here. While not a new concept at all, encouraging artists to painstakingly detail all aspects of their work so others can either recreate or improve upon it allows for said artists to not just display their creativity with how they purpose images and tools, but also to give credit for lesser-known individuals who very often are not acknowledged because said individuals do not enforce any kind of attribution.
I figured for the up-and-coming artistically-inclined, I would share some tips beforehand for when somebody wants too reveal the broader details of how something was made. There aren't entirely hard-and-fast suggestions but most of them fall under Things that make sense relative to this idea;
Don't merge. Don't merge. Unless you really have to, for when multiple layer masks are in use. Try to have as many layers as possible for your end result, so people can de-activate or modify certain aspects of your work they do not like, or wish to adjust for preference.
This is especially vital for the visible bottom-most layer of an image because it's the background. That can be changed, and should be changed to match a theme, since there are lots of instances where something is a certain colour that ruins a theme, and if someone likes an image that much said person can modify it to match, instead of working around an image they cannot easily modify.
Somewhere in your post, you should provide some basic instructions that would be usable for a majority of users and use cases. You can't tell people everything as the sky is truly the limit when it comes to this stuff but basic suggestions for people to try so they can begin exploring how to modify the image to suit is good for people who aren't terribly good with GIMP to begin with, which will most likely be a majority of your audience; People who are interested and are willing to try, but can't perform professionally with the tool.
If you take it to another level and detail effects, values and such (please, for that kind of stuff keep that hidden because many won't be interested in wading through that material for basic information should you intersperse that instead of having basics and further details being separate), then anybody who follows your steps should be able to recreate the image at-will, so they can do it their way. Is it worth the effort? Probably not for many people, but remember, you're doing it for that one person who wants to bother, and people passing by can apply your techniques elsewhere without attribution, so over time your information may end up being used elsewhere, which is nice if you wish to drop the whole art thing altogether but want to leave some things you've learned behind.
Credit where it is due. Heck, credit where it isn't due. If you used a special brush, some cool plugin or some other cool tool, skimping on these details make it less likely somebody can recreate or edit your work sufficiently.
Remember, the whole idea here is to not just present what you did, but also to teach others through your presentation. Neat effects, templates and the like are everywhere, and only select people know about them, so to include that information can make people passing by feel special being presented with this knowledge they would had not otherwise encountered if it wasn't for you.
Lots of people go through life without being given the credit they rightfully deserve, because they forfeit it. Giving these people a nod is a nice way of thanking them, by giving their work more exposure so those who become more interested in artistic endeavours are armed with information they need to perform as well, if not, better than you.
Lastly, don't feel bad if a variant by somebody else is more preferred than yours; You can rest easy knowing it was you who provided the base, and so long people treat one another kindly and continue to provide sufficient attribution for one another, being one-upped isn't something people should be concerned about if they can learn from it and help one another improve.