I've been practicing deep throating and so far have gotten very good at it. One tip I can offer is to stick your toung out on inserting it. You may not realize it but your toung gets pushed to the back of your throat and causes a block, which makes you gag all the easier.
Another is you really want to take him in your mouth. You have to want it.Phychology plays a big part in weather you're gaging or not.
Practice, practice, practice. I bought the european cock and balls and after I washed is very good, there's almost no bad taste and it has become my favorite oral practice tool. If you have a realistic dildo, practice on it every chance you get when he's not around. Do it while in the shower, that way you can not worry about gagging. So what if you gag and a little comes up, you're in the shower, it'll wash right off and no mess. With your mind off the reasults of gagging, you can focus more on controlling that reflex.(and if he catches you, I assure you he'll have no complaints.)