Summary
A Gate cannot cover a Lifeline.
Details
A Gate can only belong to an Interaction or an InteractionUse.
Tips
A PartDecomposition, which inherits InteractionUse, is represented in a Sequence Diagram by a Lifeline. This may be a cause of confusion because the Lifeline will visually be covered by Gates while these Gates really belong to the PartDecomposition that decompose the Lifeline.