I don't think you can actually use General Edit panel because it won't let you change zapata.
Use Headers->zapata.conf
Set the gains to whatever you like, issue a commit and then either reload chan_zaptel or restart asterisk. Don't forget that the gains are calibrated in decibels so they are logarithmic (double the number is 4 times the volume), valid values are -10 thru +10, unity is 0.
Best
S