The only real solution is to keep the litter box where the dog CAN'T get into it. Maybe put the litter box in a closet with the door 6 inches open. Use two rubber wedges, one on each side. The cat can get in but the dog can't.

Or move the litter to a shelf that the dog can't reach.

If you can't do either, then a baby gate is definitely the way to go. Just don't count on it to be unmovable, tall enough or not climbable. (Is that a word?)

7 month old Labs can be very resourceful!

Mary