It's nothing like Rick and Morty. They're both comedic cartoons which have some level of appeal to adults as well. That's the common thread people are picking up on. Gravity Falls is absolutely fantastic. You're completely right in saying that it's because it's rooted in realistic characters. Although cartoony, absurd things do happen, they carry much more weight than normal cartoon shenanigans. Characters go through sincere emotional growth, cliches are subverted in interesting ways, and it builds on folklore and pop culture as a foundation where other shows might use it as a crutch. Plus, the comedy is on point. Mabel: "What was the one thing I asked you not to do tonight?" Dipper: "Raise the dead." Mabel: "And what was the one thing you did tonight?" Dipper: *sigh* "Raise the dead."