The Queen's Gambit Declined: 4.Bg5 is rare and occurs in less than 1 in 100 games. From the data, you can expect that White will have a high 58.05% chance of winning, while Black should only have a 41.95% chance of winning. Because of the fact that most players probably rarely see this opening and that Black has such a low expected chance of winning, it may be well worth it to keep this variation in your back pocket. Since as White, you should have a significant edge and will probably catch your opponent off guard.