how can I eleminate the gaussian noise from eeg signal and add a gain of 18 to the filter ???

Fs = 500 , 
Wl = 55/(fs/2)
Wh = 65/(fs/2)
% I take this bandpass region betwwen the power line frq response 
[b,a]=butter(n,[Wl Wh ],'bandpass')
Freqz(b,a) 
%I know that is wrong solution because all the eeg will be removed

Answer 1

[Star Strider]

You’re doing exactly the opposite of what you want. Your filter is passing the 60 Hz line frequency and rejecting your signal and filtering everything else out. You need a bandstop filter rather than a bandpass filter.

In general, the procedure in How to design a lowpass filter for ocean wave data in Matlab? should help you design your filters.

See Remove the 60 Hz Hum from a Signal for documentation on the very problem you want to solve.