Why is the value of g greater in Terai region than the Himalayan region?

Lets begin by Newton's law of gravitation. Newton's law of gravitation states that force exerted on any massive object is directly proportional to product of their masses and inversely proportional to the square of distance between them from their center. Mathematically, 

\(F=\frac{GMm}{r^2}\)

But from newton's second law of motion we also know that  F = mg. So, comparing two equations we can see that

\(mg=\frac{GMm}{r^2}\)

∴ \(g=\frac{GM}{r^2}\) [ magnitude of acceleration due to gravity]

\(\Rightarrow g\propto\frac1{r^2}\)

Since height or radius at Himalayan region is larger than in Terai region acceleration due to gravity is smaller in Himalayan region than in Terai region.