grid-row-gap, grid-column-gap, grid-gap, are defined to be treated as legacy shorthands instead of legacy name aliases, but they must take the same grammar as the property they are aliasing, whereas CSS Cascade defines:

legacy name aliases: when the old property’s value syntax is identical to that of the new property, the two names are aliased with an operation on par with case-mapping [...]

I did not quite understand the whole discussions in #866 (especially the meeting transcript) that led to the changes in CSS Cascade for the above definition and the definition of legacy shorthands, so I am not sure the resolution is you must use legacy name aliasing when the syntax is the same. I also note that the details of the aliasing mechanism is intentionally left up to the UA for some legacy properties whose syntax is the same as the new property (eg. -webkit-user-select).