Will this circuit (for a guitar effects pedal) work?
Sorry but this isn't going to work, but well done for having a go. Firstly you have misunderstood "gain". You don't get any gain by putting a 1M pot and a 5M resistor in the signal path, that will just attenuate the signal (make it smaller). To get any gain you need something to provide amplification, such as transistors or an IC called an Operational Amplifier (Op-Amp). Once the Op-Amp has amplified the guitar signal to several volts you can then clip it with diodes - you actually have the right idea with the diodes. I suggest you Google "Guitar Fuzz box schematic", schematics for most of the well know fuzz boxes made over the last 30 years are available.