I know this question has partly been answerd here but my question is : If i use a preamp/effect processor like Digitech GSP1101, what input on an guitar amp should i use, High gain or low gain?
You should use the channel that is the quietest. Test at low volume first.
I put it that way because some amps define 'high gain' as 'the channel with the highest gain/volume' whereas others use it to mean 'for an instrument with a high output'.
You want the one that drives the amp the least; that's the one it will much prefer as you are feeding it a lot of level from the preamp.