You might want to try this, this would be both for the screen and the gallery. I will start with the screen code then gallery. Then what you need for the “Select All” button. Going to use numbers as the gallery items for this example.
For the screen-
OnVisible: ClearCollect(colGalItems, [“1”,”2”,”3”])
Gallery-
Items: colGalItems
add the checkbox to the gallery
Checkbox-
Oncheck: Collect(colNumSel, Thisitem.Text)
Uncheck: RemoveIf(colNumSel, Thisitem.Text in Value)
Realized my last message had an error on that, apologize for that.
Default: Thisitem.Text in colNumSel.Value
for the “Select All” button
Button-
OnSelect: ClearCollect(colNumSel, colGalItems);
When you hit the button it should fill colNumSel with all the items in the collection.
If you check the collections and variables area you should see the collection fill. All the checkboxes should be checked off once pushed.
I am currently away from my computer, I have the same setup you described on an app. If still not working, let me know and once I get back to the my computer. I will double check the solution on that app.