Simple Mobile apps have been very popular among FOSS enthusiasts. I've personally been using the Gallery, Contacts and the Phone app since a few years now. It's a shame that it has come to this, will be on the lookout for their forks.
I just went through my phone and this will cause a search for a new gallery, calendar, notes and keyboard.
SECUSO apps will most probably provide the notes replacement (I admit I'm really impress with the apps lineup they have).
What alternatives can you suggest to replace the other apps mention.
I pull FOSS apps from F-Droid and do my best to avoid the App Store.