Comment by tim-kt

No, the integral cannot be "expressed" as a sum over weights and function evaluations (with a "="), it can be approximated with this idea. If you fix any n+1 nodes, interpolate your function, and integrate your polynomial, you will get this sum where the weights are integrals over (Lagrange) basis polynomials. By construction, you can compute the integral of polynomials up to degree n exactly. Now, if you choose the nodes in a particular way (namely, as the zeros of some polynomials), you can increase this to up to 2n+1. What I'm getting at is that the Gaussian integration is not estimating the integrals of polynomials of degree 2n+1, but it's evaluating them exactly.