You would need to do the SHOVEL TEST.  We call it a shovel test because you basically need to do this test with a shovel or any similar lever.  You will do a shovel test when you are in a situation like this, where you may need to, or desire to leave the existing diagonal bracing.  Either the existing bracing is internal, or integral, or with its removal will deface the design or look of the gate.   You will take a shovel or lever such as a 2x4.  With a shovel you would carefully place the shovel under the latch side of the gate.  With the gate slightly ajar, place the shovel under 2 or 3 or more, strong pickets (as many as you can).  Begin to slowly lift up and see if the gate has any “play” or flex in it.  We are looking for movement in the gate that will allow us to “rack” the gate back up to the desired position.  Because likely it was once at the correct height. Or you can take your 2x4 and a few blocks, and place the 2x4 under the bottom horizontal member, on the latch side, with the gate slightly ajar, begin to slowly and carefully lift.  Be careful to see if anything looks like it is straining or sounding like it may break.  But, you are looking again for “play”, “rackability”, or flex in the gate.  We need this movement back upward, and to see if, even with leaving the existing bracing it, it will work with attaching another brace, the True Latch.    The True Latch is definitely strong enough to hold a 100+ pound gate.  But, you must try the SHOVEL TEST to see if the gate will return to an original or higher height with the existing bracing left in.   Then if so, you will need to do a PRE-LIFT routine to hold the gate up high and install the True Latch brace.  Then, release the weight of the gate to see where you are at.  However, with such a heavy and existing braced gate, we would put the gate back on the shims or blocks from your pre-lift, and then make more adjustments on the True Latch.  (We want the blocks and shims under the bottom of the gate to do some of the pre-lifting by compressive forces.  If you rely only on tension forces from the True Latch, you may start to twist the gate).  Then you should be close enough to release the full weight of the gate if you need to make a few more tweaks on the True Latch hex nuts for the final adjustment.