Can I use gentle detergent to clean a cloth mousepad instead of specialized pad shampoo?

Yes, gentle detergent works fine for cloth mousepads. Specialized pad shampoos are just mild cleaning solutions in fancy bottles for three times the price. Use a tiny dab of gentle laundry detergent (like Woolite or Dawn) with cold water, light hand-wringing, and air dry flat. Avoid fabric softener, bleach, or a washing machine on high spin.