My dog ate nitroglycerin. What immediate symptoms should I watch for?

I think Gibbs more than likely will be okay. Nitroglycerin tablets are not very bioavailable when ingested since they are intended to be held under the tongue and absorbed through the mouth. If Gibbs quickly swallowed one it might not affect him at all. If it sat in his mouth some could become absorbed, and in that case it would affect him pretty quickly. The main side effect would be low blood pressure which might not be very obvious at home, although you could see weakness or lethargy.

To be safe I'd recommend contacting the Pet Poison Helpline to see if there is anything else you need to do. http://www.petpoisonhelpline.com/