Is the Steam inventory on two different computers, but the same Steam account the same?
For example, same guns on CS:GO, even if these are two different computers, but with the same account?
Yes. The inventories are tied to the Steam account, not the computer.
However, keep in mind that if you’re trying to “share” your account with somebody… well, you shouldn’t. It’s against Steam’s TOS and can create a lot of problems if you’re trying to do things on both computers simultaneously.
But if it’s just you, and you’re using Steam on two different computers (at different times), your Steam inventory is the same on both computers. Get a new gun on one and you can use it later on the other.