I've attached two of the 36mm x 9mm magnets to my driveway picket gates. The magnets are screwed into 50 x 50mm treated pine laths attached to a couple of short star stakes. On the gates themselves I have attached two Aldi steel mending straps.
When I open the gates fully, the mending straps touch the magnets, holding the gates in place.
These magnets are very powerful. and will hold the gates open even in 100kph winds.
I recommend these to anyone having similar issues requiring strong magnets.