Phyrexian Gremlins

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.

Only prevents the artifact from untapping during untap step. Ones that untap during upkeep are not inhibited.

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.

The effect does not end if the target stops being valid. For example, if it stops being an artifact.

Can target a tapped artifact.