Previously it was tagged `compacted` which is "A mixture of larger (e.g., gravel typically 20mm) and smaller (e.g., sand) parts, compacted (e.g., with a roller), so the surface is more stable than loose gravel." per https://wiki.openstreetmap.org/wiki/Tag:surface%3Dcompacted

You'll also see on the wiki that many roads signposted as "Gravel Road" in Australia would be tagged surface=compacted and not surface=gravel.

The `gravel` value is documented as "This tag has very large meaning range. Used for cases ranging from huge gravel pieces like track ballast used as surface, through small pieces of gravel to compacted surface." Typically I'd use surface=gravel for larger railway track ballast type of surfaces, then for fine gravel use surface=fine_gravel https://wiki.openstreetmap.org/wiki/Tag:surface%3Dfine_gravel

For a highly landscaped area like here it seems unlikely to be surface=gravel and surface=compacted seems more likely.

OSM tag values can't always be literally interpreted because this interpretation varies a lot among geographies.