It's unsafe to use a pressure-mounted gate at the top of stairs because if someone falls against it, it could come loose and the person (or child) could fall down the stairs. Try attaching a piece of wood, like a 2X4 to your banister with zip ties, and mounting the gate to that. Or use one of the banister kids (Kidco has some, I believe). You could also put a pressure gate in the doorways on either side of the stairs, effectively blocking off all access to the stairs without needing a gate at the top of them (we did something similar to this to avoid drilling into our banister).
Mom to a spirited, red-headed, former 28-weeker 10/2009 and a more mellow monkey 12/2013.