I can understand your feelings. The guilt is most likely the result of the conflict between your obvious arousal of the "glorious mess" and the remorse you felt over making your wife gag. I do the same thing sometimes if I accidentally shoot in my wife's eyes but still get very excited over the mess. You get turned on but feel bad about her distress. Have to mirror Btdoug on this, talk to your wife about it. Tell her you felt bad making her gag but at the same time that was one of the sexiest things you have ever seen. Odds are she will laugh about it, you will feel better, and one of you, or both, will suggest you need more practice to get your aim right.
A bit of advice on the gagging. If she is used to taking your loads while giving a bj then it probably is not the quantity that made her gag. It probably was just the angle allowed some shots to hit the back of the throat. You can't do much for shots that ricochet off her tongue. But if she is kneeling and she tilts her head forward just slightly with her tongue out you can stop most of those straight to her throat shots.