M.R.
My son hit this stage early.
For a while, doubling up on the gates (stacking two of them per door way) worked--- until he realized he could reach through the little fencing and undo them that way.
We finally ended up putting simple alarms on all the doors and windows in the house. (These cost about a dollar each and often come in multi-packs.) They simply stick on to the surface---for example, we stick one side of it on the door and the other side on the door frame-and there's a basic on/off switch.
Now we can hear when he moves from room to room in the house and we can often check on him. This is especially useful when he tries to get up in the middle of the night. An alarm goes off and one of us can get up to see what he's trying to do--- normally it's going to the bathroom--- but we've also had issues with him getting in the fridge and pantry.
Other than that, your best bet is to minimize how much you own, put simple hook and eye latches on doors that he shouldn't be using ever (such as outdoor doors, closets, etc...) and install anti-tip devices on all shelving/dressers/and climbable objects.