If the Gremlins somehow become an artifact and then tap themselves, they can never be untapped during the untap phase. They must be untapped by an external effect.
2004-10-04 • wotc
Only prevents the artifact from untapping during untap step. Ones that untap during upkeep are not inhibited.
2004-10-04 • wotc
The target artifact can’t untap if the Gremlins and the artifact are tapped during untap, even if it is your artifact, and you plan on untapping the Gremlins.
2004-10-04 • wotc
The effect does not end if the target stops being valid. For example, if it stops being an artifact.