Using a hairdryer or similair type of heat producer and an old Credit card or (Cataloy -Filler) Plastic Spreader. Heat will melt glue/resin and plastic card will scrape it off but don't push too much with the card, could scratch paintwork and for more severe glue get a de-greaser, from Halfords or any other motor factor, and clean cloths, this should return the tailgate back to its original state. Even buff up with T-Cut to give it a smooth finish I used white spirit to clean up my spoiler but the paintwork went white so t-cut to the rescue again! Dek