- Should I raise it as an issue on their github?
I would prefer an issue on the GitLab instead of a private email since there will be a public record of the problem if the author decides not to fix it, but an email would be fine too.
What wording should I be using? I don't really want to get in a long fight of "Someone is wrong on the internet."
I think the main thing to focus on is the potential for confusion. They want to share their software; that's great! Let's make sure that people who receive that software are given a clear, consistent set of rights.
It seems the author really didn't want to use a FLOSS license. Tell them that the use of a FLOSS license alongside FLOSS-incompatible terms makes it very unclear (and legally unsteady) what terms actually apply to the project. This doesn't need to be a controversial discussion: simply point out that they clearly want to restrict commercial use or derivative works, but the license file they've chosen allows both of those things! Their project contains a README that says one thing and a license file that says another; these should be put into agreement.
(In particular, you can point them to the GNU Project's article on selling free software to demonstrate clearly that the GPL allows commercial reuse. You can also point to What is free software? to show that all FLOSS licenses allow derivative works.)
Since the README terms and license-text terms are in conflict, the author ought to choose which they actually want.
It seems like they would be much happier with a NC- or ND-clause Creative Commons license. That might not be a perfect fit either, since they may want a copyleft source-sharing requirement (based on their choice of the GPL). Creative Commons has a ShareAlike clause, but that doesn't require sharing the source code (i.e., a downstream distributor can choose to share only a modified binary under ShareAlike terms and keep the modified source private).
Alternatively, you could try to talk the author out of the need to restrict commercial reuse or derivatives, and simply let their software be normal FLOSS. Instead of a no-modification rule, why not rely on the GPL's requirement that modifications must be shared when distributing the software? It's a win for the author, since they can get improvements from others for free. Instead of a no-commercial rule, understand that the GPL is already a natural damper for traditional commercial resale, since competitors can immediately get changes from one another and incorporate them into their own version of the product. Why would someone buy the mod when an identical version is available for free?
As noted in Are licenses affected if the author adds conflicting clauses alongside?, the GPL explicitly gives downstream redistributors the ability to strip further restrictions:
If the Program as you received it, or any part of it, contains a notice stating that it is governed by this License along with a term that is a further restriction, you may remove that term.
You should also inform the author of this, as further evidence that the GPL isn't the license they meant to use (unless you convince them that their additional restrictions aren't necessary after all).
A sample bug report:
I noticed that this project has a notice in the README that [commercial reuse/modification] isn't allowed. I'm happy you you chose to publicly share your mod (thanks!) and I'm willing to follow those terms, but it's confusing to me that you included a copy of the [L]GPL, which is license that explicitly allows [commercial reuse/modification]. (See [Selling Free Software / What is free software?] from the author of the [L]GPL for more information.) Since the [L]GPL is a widely-used license with well-understood terms, I'm concerned that your inclusion of this license can cause confusion alongside when placed alongside the terms in your README that conflict with the GPL's well-known mechanics.
You might not think this a big deal: it's probably clear to you what you meant. But it might be confusing enough to other users to cause serious confusion about what people are and aren't allowed to do with your software. In particular, the text of the GPL says:
> If the Program as you received it, or any part of it, contains a notice stating that it is governed by this License along with a term that is a further restriction, you may remove that term.
That means that by placing the GPL alongside your restrictions against [commercial reuse / modification], you're actually giving people the legal right to strip away those restrictions! I'm sure that's not what you meant to do(!), but that's the legal reality of how the licensing of your project stands right now.
I think there's two fairly easy fixes to choose from:
License your software under a Creative Commons license with a [Non-commercial / NoDerivs] license. Creative Commons licenses are widely-used, legally-sound licenses written by lawyers, and the [CC-BY-NC] license or [CC-BY-NC] license might suit your needs more precisely that the GPL. However, Creative Commons explicitly discourages use of their licenses on software, since it's often not compatible with other popular software licenses (making it difficult to reuse code with yours) and it doesn't require sharing source code on derivatives like the GPL does. You can use a CC license with a ShareAlike clause (like CC-BY-NC-SA) but still doesn't require sharing the source code -- it only requires share-alike terms on whatever the downstream distributor choses to share, which could be only the compiled binary.
Alternatively, you might think about fully embracing the GPL.
[Commercial resale of [L]GPL-licensed code isn't very likely anyway, since it's freely available from you, and any modifications a downstream seller makes you can immediately pull back into your own project, making the seller's paid version only as good as your own version.]
[If you're worried about modifications, don't be: since the [L]GPL requires people to share their modified source when they share a modified version of your mod, you can use those modifications yourself. In other words, allowing people to make changes to your [L]GPL software is essentially a way to get improvements for free.]
Which path you choose is up to you, but I strongly recommend fixing the conflict in terms between your README and license text. Someone could easily use your software in ways that you did not intend due to the confusing conflict of terms. You can avoid this by making sure there isn't any such conflict.
I'm happy to answer any questions you have. Thanks again for sharing your mod.