A lot of baubles go into the charm-slot. Since there actually is a special slot for magnets in the curio inventory now (the GTCEu Magnet slot) it would be great if this magnet could also go in there and thus free the charm-slot for other baubles.

What exactly do you mean? For what Minecraft version is this, for Baubles on 1.12 or for Curios API on 1.14+?

... the GTCEu Magnet slot ...

What is that?

Looking at the Curios API wiki, I don't see any mention of a magnet slot type.

If GregTech adds a slot for their magnet, that's up to them. Any mod can add an arbitrary Curios slot. I can't add tags for any mod which adds a new magnet slot.

Curios has common slots for common accessories to avoid exactly that. The charm slot, which Simple Magnets uses, is such a slot.
If you (or some modpack) want to add a magnet slot or just more slots, you can do so through a datapack. The default for Simple Magnets will be to use the charm slot as that seems to be intended by Curios.

It doesn't seem to make much sense for a magnet to go into the charm-slot though. What points to that being intended? Also even those common slots are only there if there are mods using them.
But it's your mod. Thanks for taking the time to answer. :)

Since one of the features of Curios API is "Slots are only made on-demand. There are no slots included by default, all slots are created only as needed. This reduces instances where one or more superfluous slots are present without any suitable items to go into the slot." the CTGEu Magnet might only be there if Gregtech is installed. I did not know that before making this post.
Still, I think you could also activate and use that slot, even without Gregtech installed, or make a generic magnet slot, couldn't you?

Here is a Screenshot of the slot: