It's usually short for "gold standard", as in the highest level of achievement one measures against to see how good they are. As u/useful said, they are usually data sets/labeled data hand-made by humans with expertise in the field in question. So gold (standard) syntax trees are those curated and labeled by linguists. I haven't read this paper, but I can only assume they mean that training on hand-labeled syntax trees dramatically improves the results of their system over the common practice of letting the system come up with its own labels.

Natural language processing (NLP) is a field of computer science, artificial intelligence and computational linguistics concerned with the interactions between computers and human (natural) languages, and, in particular, concerned with programming computers to fruitfully process large natural language corpora.

A community for discussion and news related to Natural Language Processing (NLP). Natural language processing (NLP) is a field of computer science, artificial intelligence and computational linguistics concerned with the interactions between computers and human (natural) languages, and, in particular, concerned with programming computers to fruitfully process large natural language corpora.