Not sure if this will help, but some thoughts:
I was really scared to get in the back strap on the Blast (similar to the Gecko) at first once I was planing. Felt loose and intimidating. I would turn upwind to slow down, which weighted my back foot and made it really difficult to move it.
Since then I've been learning to start planing with a little bit of downwind angle, which helps unweight my rear foot so that it can slip into the back strap earlier. It also helps to weight the harness lines more. You may want to see if your harness lines are too short? Hard to diagnose without a video.
Another thing to try is to not be in the harness at all when you are planing. You should be able to sink your weight forward and low and get your front leg pointing forward, freeing your rear foot to get into the strap. Once there, you can then harness up. This is the active/more athletic way of getting into the straps, and I do it all the time if the winds are marginal and I have to pump onto the plane. In fact, just playing with pumping the sail and board a bit may help give you some perspective. It's easy to freeze up sometimes and not realize you have a lot of body language/movements that can help you in various situations.