I have been using the Pocket GK app for a few years now. I find the app to accurately reproduce the song, regardless of speed played back. I had to learn about 60 songs to play in a country/pop band, and another 40 for a jazz/funk band in the last two years. Since I dont read music (and have been playing bass about 4 years now) this tool is my GO TO when I have to transcribe a new bass line. I basically learn the gist of the chords in real time, and then I can slow the song WAY down if necessary to learn the intricate (or even softer) parts.
Its also a great app when our band plays a song in a different key. I can change pitch up or down two semitones and it accurately reproduces the song in that key. Whats nice when practicing is I go for muscle memory on songs. By practicing in the key my band plays the song in, I have fewer mistakes when playing live.
I like the fact this app interfaces directly with my ITunes library. AND the developer is approachable. I had a problem with functionality deprecation in iOS 8, but when iOS 9 was released Apple fixed their issue and the app works like it did from day 1!
Finally, I play live with an epifani head and cab, and have played with a Gk head before. I feel the tones from the app reproduce the Gk tones quite well!
If you are considering this vs another - get this app. You wont be disappointed. (But be sure upgrade to iOS 9!). And, for the record - I dont know the developer nor do I have a vested interest in his nor in GKs success. This is probably my favorite app on my entire iPad.