PGI stands for Post goods issue in SAP SD. Whenever goods are issued to the customer, system does the following:
1. Updates stock in warehouse (MM data)
2. decreases the volume of stock (MM data)
3. Increases the goods sold (Sales data)
4. Updates the G/L account (FI data)
PGI process is basically carried out after picking and packing of the material for delivery.
i.e. PGI process is triggered after the delivery document is generated.