How can I close up a gap in the corner of the shower?
IF this is your only issue with this shower then you will have to borrow from the cheap hotel maintenance mandate of fixing shower corners.
That would be placing a strip of plastic (pvc) or glass along that whole corner.
This is how it works.

you get a 3&quot; by &quot;1/4&quot; piece of glass or pvc that runs up the entire corner (high enough where water shouldn't be entering the top)
epoxy/glue that to the back wall
caulk each side and the top

The only weak point after doing this will be the very very bottom.   You just have to recaulk that frequently - every 6 months or so.
In my teen years I did this for a hotel chain using very flimsy shower pvc.   It would generally give the showers another 1-2 years which in the hotel world is like gold.