set.seed(123)
n = 20
p = 3
y = rnorm(n)
x = matrix(rnorm(n*p),n,p)

glm1 = glm(y~x,family = gaussian)
glm1$deviance # == [1] 17.28482
glm1$null.deviance # == [1] 17.97548

var(y) * 19 == glm1$null.deviance
sum(residuals(glm1)^2) == glm1$deviance