Am doing some tabbing in GP5 and came across this 8th note with an X under it. What does it mean and how can I do it in GP5?

I do believe that would be a dead note.
Unless you are doing percussion on GP5
Than that is a cymbal(usually hi hat)
The first is a palm muted note, and then the other notes are tied I believe.
its not exactly palm muted chikao, although ur on the right track its when ur hand is placed over the fret board making a percussive sort of sound

as heard on linkin park Given up intro i think for a lil example
Ok heres the actually picture

It says to mute but do I mute both notes or do I just play any muted open string?

The pics blurry but it says

It's a left handed mute then... A much easier way to play that would be a downstroke and don't hit all the strings, though.