Actually, from the photo the grade is about 30%. I actually was able to start the bike up from this parking, without zigzagging and my smallest ring was then 24T. Since then I went down to 20T. For steep enough hills lower and lower gears provide no real gain for steady riding as you start to roll back anyway during these portions of the turn of pedals when you have no leverage. You can get though a momentary boost for starting or temporary increase in slope.
My challenges are nothing though compared to those for people cultivating fields in Rwanda. Here is a typical hill there used for agriculture.
Rwandan Landscape