Ajmal,
At this stage, understand what happens during goods receipt, what is the direction of goods, in which business scenarios do we have GR in SD.
Predominantly, SD is goods from your company > to customer, so outward movement of goods (GI). But when customer wants to return the goods, then inward movement of goods (GR).
A good approach would be to take a process and read about it - OTC, Cash Sale, Rush Order etc.
TW