its hard to tell if its a male or female from the picture. If you want to find out, put a know female or male in the same tank with a partition and you will find out after a couple of week if given the right condition.
This is a good starting fish for you to gain some experience in keeping and knowing the grading of flowerhorn.
Grand sumo will help fatten up your fish, but it will not give your fish a bigger or a waterhead. Waterhead is genetic. You can feed your fish the best food its entire life and if the fish does not have the gene for a big head, the head will never get big. Don't waste your money on those imported food. Give your fish the best of care and feed variety of food.
If you're looking for a flowerhorn that will have a big head/hump/kok, put more money into a higher quality flowerhorn that you save from buying those expensive food.