guys i was going through this diagram in my book when these lines came along(i am a student): Here RG(usually in M-ohm range) establishes a dc ground at the gate and presents a large resistance to input signal..... I want to ask why resistor is used? can't we directly ground it?
You can directly ground the Gate, sort of.
The Gate is where the input signal is applied. If you have a direct short from Gate to ground, the signal is shorted to ground and there isn't anything to amplify.
If your signal source has DC continuity, you can connect it between Gate and ground without any problems.
This signal source might be a transformer or inductor or similar. The DC continuity provides the bias path that the FET requires.