For example, if you tab out some chord being finger picked. In real life, each note would sound out until the string is hit again, or the finger is moved off the fret. How do I apply this holding of notes to the entire tab? I don't want to go through every single note and put a tie to where I want. I hope I explained that clearly.
You should use the Let Ring function.
Just mark the section (or entire song) you need this effect for and press "I" (or Effects->Let Ring, for GP5).