Related to gagging, some women also worry about the violence of his thrusting, particularly as he gets increasingly aroused. When he's thrusting, it's called irrumation, as opposed to fellatio. During the latter, you're moving your mouth up and down his penis. You're in charge: You control the rhythm, pace and pressure. In the case of irrumation, he's thrusting in and out of your mouth. In any session of oral stimulation, there's likely to be a bit of both, but make sure, in general, you're in charge, and remember that men tend to buck and thrust when there's some sort of stimulation (either friction or pressure) they require to maintain arousal.
As for swallowing his ejaculate, the main thing men enjoy is stimulation through orgasm. Many women stop at the point of orgasm, rather than stroking through the orgasm. Swallowing naturally provides that persistent stimulation, which is why men enjoy it. But if you don't enjoy swallowing (and many women don't), there are plenty of other ways to stimulate through orgasm and maximize his pleasure with your fingers, breasts and tongue. What's most important is to understand that men love being orally pleasured in a countless number of ways. And a little fanciful finger work, plenty of talking, eye contact, teasing and squeezing will out-lip any Linda Lovelace.
There are many, many paths to sexual gratification, and no one approach is necessarily better than the rest. In fact, you should get out of the habit of taking the same path each time. What's important is that you understand the principles of pleasure and continually find new and creative ways to attune yourself to his pleasure with confidence. Your excitement and sense of control will be the most erotic aspects of any sexual interaction.