Hairballs are the number one cause of vomiting in cats. If you are giving the hairball gel daily, and he is still vomiting, it may not be hairballs.
Purina cat chow is not a very good food. Instead of real chicken they use "bi-product meal" which is the left over parts of the chicken ground up. Then they also use corn meal. Corn is highly allergenic. Plus, would a cat ever eat corn in the wild? Corn is a cheap filler. He may stop vomiting if you put him on a better diet. Try feeding him a higher quality diet that has "real meat" as the main ingredient and no corn fillers. You will have to go to a pet store to get a good quality food. I feed mine "wellness." So gradually change him over to a higher quality food and see how he does for a few weeks. Continue the hairball prevention too.
If it that does not work, there are a number of other things that could cause the intermittent vomiting.
He could have a gastrointestinal foreign body, such as a piece of string, that is causing the intermittent vomiting. This would have to be diagnosed with x-rays.
He could have inflammatory bowel disease.
He could have a bacterial or parasitic infection. The first part of the diagnosis would involve a fecal test. Then he may need blood work. It can be treated with oral medication.
He could also have a metabolic disease like kidney, liver, or hyperthyroidism.
All of these would have to be diagnosed with blood work.
He could also have food allergies. These can develop over time. He could be allergic to the corn in his cat chow.
See if he improves on a higher quality diet. If he does great. If he does not, then he will need to see a vet and have more diagnostic work done to figure out the cause. Vomiting once a week, with out other symptoms, is usually not too serious so do not worry.