Yeah, the negative Gs with flying is different. Different parts of you are being accelerated differently to other parts and to planet Earth, so the biological sensation is negative Gs, but the physics measurement is all still acceleration, period.

Also, I don't know how I keep missing questions. To answer your gravity question... Basically, space and time are connected. You can't experience them separately. If you change how you experience one, you change how you experience the other--they call it "spacetime". This is essentially where matter and energy exist, and where they do all their stuff--it's the "fabric of the universe" everyone is always talking about. Gravity "warps* spacetime.

We're getting a little in the weeds, but we know this warping happens by looking at how light travels. We assume light travels in straight lines on space-time without gravity. Unlike regular matter, you can't really push or pull light with a force--it can't accelerate because it is constant. The only way to make light turn is by curving what it is traveling the space-time it is traveling on. Massive things with a lot of gravity deflect light, so the conclusion is that gravity is curving space-time. The result for normal matter following this curved space-time is time dilation.

In terms of actually measuring this, it looks like things get bent in pictures and in telescopes. If you ever see the "gravitational lensing" photos from Hubble or James Webb telescope, that's what we think we're seeing--light being deflected by gravity. It's called "lensing" because someone figured out that gravity bends light like lenses--so we can use them like lenses in a giant telescope.