Well let's take your example of gravity. We know a lot about it. Like you said, we've put stuff on other planets. And we have satellites in orbit. Amazing. But even with that we're still theorizing about exactly how it works. And we definitely can't recreate it.
It's possible that there are hard limitations on our ability to recreate some of the very fundamental phenomena we experience in everyday life. Intelligence may be among them.
The only theoretical hard limit that makes sense is a limit on information. If you can't transmit information from A to B faster than C, then yes you've got a hard limit.
Every other natural phenomenon that doesn't obscure infromation that way can be figured out. There may of course be soft limits like it taking a billion years to figure out.
It's possible that there are hard limitations on our ability to recreate some of the very fundamental phenomena we experience in everyday life. Intelligence may be among them.