As the GK-5 is a magnetic pickup, it will only work with steel strings.
Typically, a guitar with nylon strings will not be compatible with a Roland BOSS GK pickup like the Roland GK-3, or BOSS GK-5. So unfortunately, a GK-5 will not work for a nylon string classical guitar, for example.
An alternative for nylon string guitar players, that want to trigger a Roland/BOSS guitar synth, are guitars/basses that have alternate ways of tracking a string. For some manufacturers, such as Godin, they manufacture the Multiac SA model nylon string guitar, or the XTSA model electric steel string, guitar.
These guitars are equipped with a bridge piezo pick-up systems, built into the bridge, or bridge saddles, for tracking of each separate string, via piezo contact. In this case, the bridge becomes your hex-pick up.
Another option for nylon string electric guitar players, is to instead use our BOSS SY series guitar synths: SY-1/SY-200/SY-300, which all allow for simple 1/4" TS mono cable input from any guitar/instrument.
These would not be able to provide individual string tracking or alternate tunings, but are a great alternative for synth sounds.