You can still turn the gain control, and possibly could even superglue the knob back on but I just have not done it. The item "DigiTech GSP1101 Multi-Effects Guitar Effects Pedal and FS3X Foot Switch" is in sale since Saturday, December 29, 2018.
This item is in the category "Musical Instruments & Gear\Guitars & Basses\Parts & Accessories\Effects Pedals\Processors". The seller is "adembiczak" and is located in Glastonbury, Connecticut. This item can be shipped to United States, Canada, United Kingdom, Denmark, Romania, Slovakia, Bulgaria, Czech republic, Finland, Hungary, Latvia, Lithuania, Malta, Estonia, Australia, Greece, Portugal, Cyprus, Slovenia, Japan, China, Sweden, South Korea, Indonesia, Taiwan, Thailand, Belgium, France, Hong Kong, Ireland, Netherlands, Poland, Spain, Italy, Germany, Austria, Bahamas, Israel, Mexico, New Zealand, Philippines, Singapore, Switzerland, Norway, Saudi arabia, Ukraine, United arab emirates, Qatar, Kuwait, Bahrain, Croatia, Malaysia.