In philosophy, there are two classes of models about time. In A-Theory, time is primarily about tenses and the flow of time. Past-Present-Future are definitional. This certainly corresponds to how we experience time.
However, in Relativity, Spacetime is one thing. Time is just one out of four dimensions. For that matter, people moving relative to each other don't have exactly the same time axis. (This is why events get further apart, but spatially closer together in the direction of travel.) The equations of physics, even before Einstein, work equally well in either direction. This is a b-theory of time, where the flow of time is an experience we impose on something that is fundamentally one solid thing. (Perhaps related to how the accumulation of experience involves the growth of entropy.) B-theory is often referred to as "block time". The universe as a 4D sculpture.
Now what does this have to do about the Ralbag and Omniscience?
In b-theory, there is a truth value to "John will buy a car tomorrow". What we don't have today is knowledge -- we don't know whether he will or he won't. But one of those two is true right now, already.
In a-theory, there isn't. It's simply not a fact yet. We don't just not know, it isn't part of reality.
The Ralbag's theory of time is an a-theory.
There are less discussed, theories c-theories of time (sometimes labeled "c-theories") that would fit the Ralbag's words. Such as if he believed in a growing block time -- the past and present are real and true, but not the future. But in any case, the Ralbag does not have a b-theory, his is not a block time in which all points on the timeline are equally real.
The Ralbag believes that Hashem knows everything. But claims about the future don't exist -- they aren't part of "everything".
(That said, I have my own problems with the Ralbag's definition of Omniscience. It places Hashem within time by assuming He Knows something after my decision that He didn't know before. But G-d is למעלה מן הזמן - "above" time; the Creator of time rather than one of us who live within it. Which in itself answers the paradox of Omniscience and free will. G-d doesn't Know now what I will decide tomorrow. But not because He doesn't Know, but because He doesn't do it now, or at any other point in time.)