Actually, the wrong question was asked in the thread title...the issue really is not if one "can" keep them on gravel, but should one. Putting the best interests of the fish first.
It is also impossible from the photo to know the actual grain size, as we don't have a scale. But if the grain size is larger than river sand to the extent that the fish cannot filter feed, then it is completely inappropriate. And all species of Corydoras are filter feeders because this is programmed into their DNA. Sand is frequently found in the gut of Corydoras fish, according to Ian Fuller; "Also complete bodies of ants termites and a host of other similar creatures. I am pretty sure the sand aids the breakdown of these animals."
Some members here respect Ian Fuller as one of the foremost Corydoradinae authorities. A new described species was recently named in his honour, Corydoras fullerii Tencatt, Santos, Evers & Britto 2021. Following are direct citations from Ian [no idea how to copy over videos, sorry].
May 28 "OK folks. With all the discussion about substrate. This is a video clip of a C. ornatus feeding on tubifex in the sand substrate. If the substrate particle are too large to be able to be taken up by mouth and ejected out through the rear of the gill cover then it is not suitable.
To avoid posts from inadvertently encouraging the use of inappropriate substrate, post with images showing anything other than fine smooth grained sand will not be accepted.
The same will apply to any posts defending the use inappropriate substrates. in both this and its sister group Corydorasworld Members Group page.
Ian"
Later in the same thread:
"It seems my advice was not headed and my delete button is found to be working very well."
May 31 "This will not be everyones cup of tea, but it bothers me not one iota. As of now pictures of Corydoras over inappropriate substrate will be rejected.
Our message to people wanting to keep and even breed Corydoradinae Catfishes is to do the right thing and provide what is the right substrate for their Corys, "Natural Sand"