the word "gravel" is used in the product description and name, therefore it is unsuitable for axies.
children's play sand is a suitable substrate, or none at all. that gravel may be fine, but you must remember that it is still gravel, abrasive and potentially life-threatening for a axie. they have no scales, so having to crawl on that substrate all day would be like dragging your bare belly on coarse sandpaper.
you must remember that axies are not fish and as a axie keeper, you must home them accordingly.