The bracha of Hatov v’HaMeitiv in Bentching is rabbinical, and was composed by Rabban Gamliel and his bais din as a thanks for the miracle that the bodies of those killed in Beitar, still were not decayed 15 years later when they were allowed to bury them. What is the connection between this that it is inserted into Bentching?

The rabbis saw it important to institute here the bracha: HaTov v'HaMeitiv, which is the bracha we should make whenever we hear good news that affects several people. Believing that everything God does is for the best, we really should be making this bracha a lot. Unfortunately, we take most good things for granted. Our sages took the opportunity of the murdered of Beitar, which was a terrible historic national tragedy (basically the end of the Bar Kochva rebellion & finalizing our exile), to teach us that as we thank God for our food, and for the Torah and the Land of Israel (that's basically Birkat haMazon), we should also thank God even for the smaller things, and even for the relatively small good which comes together with a significant large tragedy! To see God's kindness even there, and how much more so in the obvious goodness mentioned in the rest of bentching.