How do I help my baby accept food with texture?

My baby was on only breastmilk for the first 7 months of his life. Then I added baby food to his diet. Now he is 9 months old. Sometimes I thicken his baby food up with oatmeal cereal and he takes that just fine but if I try to give him anything with even the tiniest lumps in it he gags and eventually throws up so I have to make sure that it is completely smooth if I am going to feed it to him. I have tried even the gerber third foods thinking the stuff in there would maybe be mushy enough and small enough for him to handle but nope he gags then throws up. My doc told me to try those cereal puff things that way there is no danger of actual choking but the same thing happened. I was hoping to find someone on here that dealt with the same type of thing. I am fine with feeding him the smooth stuff for a while since he has only been eating food for two months but wondering how to make the transition easier on him. I feel mean if I give him something knowing it will make him gag.