Great Dane breathing heavy, pacing, eating grass. Could it be bloat?

These symptoms may be his way of telling you he is uncomfortable. Pets sometimes eat grass if they have an upset tummy as well (pine sol or otherwise). So, if he has some stomach discomfort, gas, or feels nauseaus he could have these symptoms. Danes are also a breed that is more likely to develop what is called a GDV (gastric dilitation volvulus or bloat). This is where the stomach flips and this is an emergency is this happens. Unfortunately the early symptoms of this more concerning problem can also be what you are reporting (although that problem can also result in gagging/wretching, attempted vomiting, and weakness). It is good that the gum color is normal. If he doesn't seem to be feeling better soon or acts worse then i would recommend he be seen just to make sure this is not something serious. I hope this helps and thank you for using Petco Pet Education Center, formerly Petcoach!