They can’t relicense it without permission, what they’ve done right now is fully allowed under the GPL: they’ve started charging a fee for the binary.

The GPL allows anyone to:

1. Offer the source for free to anyone.
2. Charge a fee for a copy of the source.
3. Charge a fee for a binary, while offering free, or for a nominal fee, access to the source.
4. Not offer the source to anyone who hasn’t paid.

So they’ve switched from case 1, to case 3 (and maybe 4).

At any moment they can keep any part of the apps, or any of the apps, back in case 1, while asking for a fee to access the other versions (like the “Pro” ones). If they did so, then anyone who paid for a version, could ask for the source and decide to publish it for free as in case 1… or not, it’s the buyer’s choice now.

The “kind of dick move” effect, is that F-Droid was carrying a “Simple Gallery” version which now shows a popup requiring people to pay.

Going forward, F-Droid is carrying the Fossify Gallery, which is the same source with changed branding and no popup. SMT could still play nice and offer a “non-Pro” version of the apps that could get carried by F-Droid, which could lead someone to pay for the “Pro” version. Equally, random people could pay for the “Pro” version and make the source available to Fossify for free so they can port the changes, then have the resulting app carried by F-Droid.

So it’s up to the new SMT owners whether they want to play ball, play hard to get, or get forked away.