A long-throw gate lock or key-lockable gate latch beats a padlock-and-hasp for daily use — key operation both sides, no exposed hasp to attack. Side gates are a favourite burglar route to hidden rear doors, so lock them like a door.
A weatherproof surface-mounted lock throwing a long bolt into a frame keep, operable by key from both sides — the standard secure answer for timber and metal gates of varying thickness.
Yes — an open side gate gives cover to work on your rear doors unseen. Keep it locked by default; it's the cheapest layer in the whole security stack.