First, check that it is really is the gate that's the problem, and not the hinges or whatever the hinges are attached to, or heave/subsidence in the land.
If it's a metal gate, it's unlikely to be the gate itself that's the problem.
If timber, measure the diagonals of the gate, and if they are much the same, it's not the gate itself, but something else - probably the hinges worn or loose.
If the diagonals differ by more than, say, 10 millimetres the gate needs repaired - unless it's badly rotting, which would mean replacement.
Lift up the end furthest from the hinges, waggle it up and down a bit, and try and see if you can spot which joints are loose - you might find there's an obvious fault that could be easily fixed.
If there's a general 'looseness', and you're a bit handy with basic carpentry, and have the time, you could remove the gate, dismantle as much as you can (the loose joints should come apart easiest), and reassemble with new glue and dowels - but make sure you keep it square! And do it inside in this cold weather, and keep it flat on the floor!
Otherwise use stoneyboy's advice, but use one block at the end furthest from the hinges, and make it about 10 millimetres higher than you want to finish up with, as the gate will drop slightly when you remove the supporting block after fitting the bracing.
Additionally, you could add an extended stile and support at the hinge end - something like the Yeoman gate. If you Google "britishgates.co.uk" and search for "field gates"
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