I did the Rat Run (linked above) last weekend. Started on Friday morning, finished just after lunch on Sunday
Some of the area was absolute muddy hell and I was pushing on quite a few bits (up, flat and down), particularly around Cranbourne Chase. New Forest area was running mostly quite good, particularly if you stick to the bigger gravel tracks rather than the muddier bridleways. In fact the bigger gravel tracks are pretty much the perfect terrain for a gravel bike, they’re really rather lovely
I rode it on a rigid 29er (Mk1 Solaris with carbon fork and jones loops, 29+ up front) but it would be OK on a gravel bike if you are happy to take it easy on some of the rockier descents