The equation of a line is:
y = mx + b
where m = the slope of the line and b = the y-intercept. Your equation is:
G = 4000+ 300d
Rearranging:
G = 300d + 4000
Here we use d instead of x and G instead of y, but it is the equation of a straight line with slope m = 300 and y-intercept = 4000. The initial value occurs when d = 0:
G(d=0) = 300*0 + 4000 = 4000 (start, day 0))
G(d=1) = 300*1 + 4000 = 4300 (day 1)
G(d=2) = 300*2 + 4000 = 4600 (day 2)
etc.
The rate of change is just the slope, m = 300 tonnes per day, which you can see when you look at the values of G for d = 0, 1, and 2.