Antonio Gates is in his 10th year in the NFL so he has a good idea of you can and can't get away with on the field. The referees (replacement or otherwise) can't make every call on the field so every player tries to take advantage of that.
For example, players can't set a pick like they do in basketball but you can accidentally cross a defender's path.
But, no, Antonio Gates, you can not drop the defender like this.
Pass interference. Philip Rivers did a lot of complaining about it because it erased a big third down play.