Yeah, like Chad said, the differences are small, but noticeable. A gravel bike (as compared to a CX bike) usually has a higher BB drop (BB is closer to the ground) and a longer wheel-base, both of which should make it more stable. You may also see differences in head tube and seat tube angle. But, the differences will vary depending on the specific make and model. For example, my Salsa Warbird (“gravel race bike” per Salsa) has 5mm more drop than a Trek Boone (CX), but only 2.5 more drop than a Kona Super Jake (CX) (I just happen to know this because I compared them when I bought the ConflagrationAvian).
I recently heard on a podcast with a pro CX racer (I think it was Groadio with Amanda Nauman) how many manufacturers are dropping CX lines and having their pro CX teams ride their gravel bikes in elite CX competitions. This is not great for CX, as the bikes are a little different, but apparently close enough that manufacturers are eliminating CX lines.
You are right in thinking they are very similar. And, the comparison to 90’s mountain bikes is apt! We are even starting to see various suspension options for gravel bikes. And, GCN has had at least one video talking about this very fact; it seems we are just repeating the evolution of the mountain bike in many ways.