GD means Group Discussion and PI means Personal Interview. This generally is conducted in various institutions and after given entrance exam. Like in MBA admissions, after the exam as other selection rounds, GD and PI are conducted.

Examples of Personal Interview Questions are:

Tell me about yourself?

What is your greatest weakness?

What is your greatest strength?

and examples for topics for GD are:

Corruption

Gender Inequality/bias

Internet

etc..

Let me know if you want to know for any specific exam.

All the best!!!