Full Idea
Gravity is not included in the Standard Model because we simply cannot study it using particles.
Gist of Idea
The Standard Model omits gravity, because there are no particles involved
Source
Gavin Hesketh (The Particle Zoo [2016], 09)
A Reaction
I'm guessing that Einstein describes how gravity behaves, but not what it is.
Book Reference
Hesketh,Gavin: 'The Particle Zoo' [Quercus 2016], p.233
Related Idea
Idea 21177 String theory offers a quantum theory of gravity, by describing the graviton [New Sci.]