Clearest proof is the warping of light as it bends around massive objects.
https://en.wikipedia.org/wiki/Gravitational_lens
More proof: GPS works. General Relativistic corrections are ESSENTIAL for GPS to work.
GR has made MANY predictions over time, and so far seems to have a 100% success rate. This cannot be ignored.
The notion of "Flat Space" cannot be defended on experimental grounds.
There are several sources that can explain General Relativity to a layman.
Here is one of them: https://profoundphysics.com/general-relativity-for-dummies/