For reasons of "nowhere better to put them at the time", the gap properties currently live in the Align spec. (Alignment can create gaps via distribution, so it wasn't unreasonable for explicit gaps to live there.)

But now we have a Gaps spec, which currently just defines the *-rule properties, but which depends on gaps to exist. We should just move the gap properties to Gaps (and possibly slightly retitle the css-gaps spec to "CSS Gaps", rather than its current more-specific title of "CSS Gap Decorations").