This is a major issue with all GDAs. There is no evidence of the selectivity toward muscle vs fat cells. I would avoid them during the ULC portion of carb backloading. They may or may not be helpful during very low carb or fasted periods. However, Kiefer states that any modest increase in insulin during these times wouldus from a predominantly fat burning mode, to using carbs as our primary source. GDAs may have a similar effect.
Most of the manufactures reccomend taking before a high carb meal. In theory this would result in less insulin being produced. The question is, what is the benefit? Glucose will be clear either way (assuming a healthy trained individual). They may help improve overall insulin sensitivity, but then again a low carb dieter is already insulin sensitive.
Post exercise there IS a non-insulin method for glucose to prefentially being taken up by muscle cells. So I wonder if these products would cause more harm than good during this window. In other words, we would be introducing a non-selective mechanism during the only time where there actually is selectivity.
Of all the the uses, I think just before bed may be the best. The would help clear any remaining extra glucose and set up for a good night-time fast.