The protein comes from things we eat, just like the gorillas.
Vegetables have protein in them too. The amount of protein per 100g there is 2.1g in mash, 1.7g in parsnips , 0.9g in carrots, 5g in peas, 2.8g in broccoli, 1.9g in cauliflower, 3.3g in sweetcorn, 7g in a serving of Yorkshire pudding, 9g in veg gravy, 5g in pasta etc...
Now if we reduced it from 100g each to the suggested serving sizes thats about 18-20g of protein. It's not like the food we eat are magically devoid of any protein. You are recommended to eat 15-30g of protein per meal so perfectly in recommended range. Veg is also less calories, provides more vitamins and fiber and is easier to eat much more of filling you up (and its cheaper on the pocket).
I have certainly never felt hungry afterwards.