I have this issue from many days. Actually all the photos that are older than 24 hours show up as Grey in all the apps in my Phone.
I saw many issues with the gallery but my gallery is fine everything there is good. But like i use instagram whenever i try to upload any photo which i took 24 hours ago i can't find it as everything is grey there and even i click the grey boxes nothing happes. Same is with every app .
If it only on Instagram you should ask them why this is happening (i think Instagram just saved a link to the pictures on your devices but not sure, only read somethingabout it)
Vous aimez la photographie ? Si OUI, cliquez ici : https://eu.community.samsung.com/t5/parlons-photographie/bd-p/photo-club-forum-fr You like photography , if YES, Click here : https://eu.community.samsung.com/t5/photography-talk/bd-p/photo-club-forum-uk