What that means was that in his body, the presence of added GH was extremely effective in adding "growth". Much more so than in other people.
How do we know? Well his massively changed facial features are the clearest sign. GH hyperresponders faces basically become exaggerated charactertures looking almost castoonish. One look at how Dallas's forehead, nostrils, nose, ears, chin and eye sockets changed will tell you that. It's a great trait for body building. Dexter has it, Rolley has it. Kai too. Problem is that many who have this will also get rapidly growth in certain organs. Heart is especially prone. So these dudes massively increase risk of heart attack or a "floppy heart" which is so enlarged that is no longer pumps efficiently. Add stims, or large body mass, or diuretics and it's a dangerous mix.