the cap i think you are referring to is for the charging valve of the mlg?
having a lot of experience working on landing gear i would have considered this as a cover to prevent dirt entering in the valve. the charging valve itself has a special operation. it needs to be loosened twice before you can service the gear.
regarding dispatch for sure you can fly without this cap so raising a HIL was the right thing to do.
you can also see them on brake bleeding ports, door accumulators as well as the resevoir charging port. there are a lot more.
hope this helps