Why is communing with the Dead forbidden for Jews but I guess gentiles do it all the time at graves? Is it okay to go to a grave and let’s say ask forgiveness from the departed? Or thank the departed, or otherwise speak?

There are many intricate details on the issue, but in general, speaking to the dead person can include many serious prohibitions, and it clearly is not the way of Judaism. When we go to a grave, usually on the Yahrtzeit (annual date of death), we speak to God (!) and ask Him to forgive us or grant us certain requests for the sake of the good deeds of the deceased. The significant exception, is when a living person offended or hurt someone & needs to ask forgiveness, & the offended person has already passed away, the sinner gathers 10 people around his grave and asks forgiveness from God (!) for the sin that he did to that person. Note, even he speaks to God, not to the dead, and in fact, if the deceased is buried in a different city, you may gather the 10, even not by the grave, for we aren't addressing the deceased anyway. The Rambam even holds that after one's burial, you don't return to the grave even on the Yahrtzeit, for Judaism stresses life and not death! The way to perpetuate one's beloved after their passing away, is by donating or performing some mitzva in their memory, for good actions are what life is about, and especially when we add benefits to the dead who taught us to do those deeds. I should add that at the burial itself (and some say on the Yahrtzeit, as well), tradition says that the soul of the deceased is there, and can be addressed, without any halachic problems.