If your app works best by users working through a series of exercises in sequence, having a Next Exercise link will save them from having to hop back and forth from gallery to detail.
Remember that each time a user has to
Locate one of the thumbnails that represents an exercise that is NOT completed already. (Your thumbnails will have a visual indicator of completion I'm assuming)
Click said exercise.
Complete and verify.
Close the modal (Not sure if this just closes on it's own).
Scan the list again for the next uncompleted exercise, which could be below the visible portion of the screen.
If you want to allow them to skip over an exercise, you could keep the 'Next Exercise' link always enabled. You could have 'Next' and 'Prev' so they can jump back and forth. Make it clear that 'Verified' is either completed or not if they navigate to a previously completed exercise.
If completion in a sequence is a requirement, keep it disabled until the current selection is 'Verified'.