His family relocated to Pittsburgh and later Chicago, where Gruhn attended Oak Park and River Forest High School.

Gruhn noticed that older and used acoustic guitars sounded better than new ones, and had the idea that vintage instruments could potentially be categorized much like zoological taxonomy.

Gruhn developed a fascination for vintage guitars and found so many good deals that he began buying and selling the instruments for a profit.

Gruhn's store played a key role in establishing the vintage guitar market[4] while also becoming one of the world's leading vintage musical instrument retailers[5] and attracting the business of notable musicians, including Eric Clapton, Brad Paisley, Lyle Lovett, Emmylou Harris, Billy Gibbons, Rick Nielsen, Vince Gill, and John Hiatt.

Gruhn utilized his idea for applying zoological taxonomy observation techniques to vintage fretted instruments, modeling identification keys after zoological equivalents and collaborating with co-author Walter Carter to publish Gruhn's Guide To Vintage Guitars: An Identification Guide for American Fretted Instruments in 1991.