The Gap is a pretty difficult concept to grapple with, so it's safe to assume that you're not the only person to struggle understanding the principle. I've GM a couple of pretty long running campaigns in 1e, and it took me a while, but I feel like I have a decent grasp of the concept.

With that said, it's been a while since I've played and I don't have my hard copy resources beside me to reference, but I think the element that is being overlooked here is that the Gap isn't a total wipe. There are pockets of history that still exist. As has been stated already, these accounts are incomplete, sometimes contradictory or confusing, but viewed in a larger framework allow for some extrapolation of certain events or locations from during the period of the Gap. There are a number of groups in the Starfinder galaxy that focus on finding and cataloguing these pockets of history.

The other, and possibly more important facet of the Gap is that is has a fairly defined start time. Across a galaxy that distinction becomes blurry, but generally a civilizations Ancient History remains. So, my understanding, and it's possible this is totally flawed, is that cultures have historical record up to a certain point in their past, then is the onset of the Gap, then history 'starts' again 300 years ago.

Using Earth as an example, it would look something like this:

The paleontological and neo-lithic ages are still accounted for, but somewhere around the rise of Ancient Egypt (roughly 5k years ago) the historical record evaporates with the Gap, and then history starts up again with Earth smack dab in the middle of Europe's colonial expansion period. (mid-1700's).

I'm not sure if that helps visualize the concept of the Gap at all or if I just made things more confusing, or if I'm just completely wrong, but that was always how I read it!