A few months ago, Wayforward released a trailer saying that Mighty Switch Force Collection would be released on GOG.

https://youtu.be/dqDoDUel_GU

The Steam version has been released on 25th July 2019, but i can't find any mention of the GOG version getting released.

Has anybody heard anything?

They(Gog) might be testing the game first on all systems, or the dev doesn't want to sell it here until steam has it awhile, but I think the more likely answer is another dev pulling a game off of being on a certain store after promising it to everyone.

It has happened before with other games, so this is probably no different.