You could essentially implement a semaphore by using an extra column in your data source but this would probably be more work than it is worth. Here is the scenario I am thinking of:
Add a column to your data source called isLocked that defaults to false
Add a rectangle to the gallery item that covers the entire item and make it clear. This is what we will use for onselect property to navigate to the edit form. Set this rectangle's display mode to the following:
If(ThisItem.isLocked, DisplayMode.Disabled, DisplayMode.Edit)
When a user clicks on an item in a gallery they are navigated to either a pop up edit form or another screen with the edit form. Either way when that item is clicked in the gallery do something like the following:
Patch(YourSPListName, ID = GalleryName.Selected.ID, {isLocked: true})
This will lock that item in the gallery for all users except the first one that clicks it. You will also need to make sure you make isLocked false again during submission or canceling of the form edits.
