Activate Dimension Gate to protect your monster from attacks or card effects by banishing it. Use Magic Planter to send Dimension Gate to your Graveyard, then activate Dimension Gate's effect to bring back your banished monster to the field.
Sends from your field to your Graveyard for cost / You draw cards
No, this card is awful for Mirror Wall. The only way to pull that off would be to pay 2000 LP to keep it on the field or activate it during your Main Phase, both of which are wastes. Dimension Gate is fine, but not Mirror Wall.