I am glad to hear that you do not think that it is bloat.
The first thing that I would do is inspect his mouth very well. I have seen lots of different things stuck in dogs teeth or wedged up it the roof of their mouth that is causing gagging.
If I didn't find anything, I do not think I would induce vomiting unless I knew that he had eaten something toxic. Hydrogen Peroxide will work for inducing vomiting though. A tablespoon or so to the back of the throat will usually work.
Very often people call to the clinics I have worked and are afraid that their dog has something caught in their throat or are choking. More often it is that the dog is just coughing. If they are getting an upper respiratory infection or have chewed on something that was irritating they may cough and gag alot. They may even bring up a white foamy substance without truly vomiting. It is because of this strong possibility of an upper respiratory infection or irritation to the throat that I would not recommend inducing vomiting. This is just because the acidic stomach contents can further irritate inflamed tissue.
If he continues to cough/gag I would recommend bringing him to the vet as he may need an antibiotic. You can use some over the counter cough suppressants in dogs. Here is a link to more information.
If hope that this helps, if you need more information, just reply to your question.
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