In the capo tables designed for printing, GKey means guitar (or mandolin) key and RKey means the real key or "concert" key, such as would be heard on a piano. In the first table (Keys Organized by Capo Setting), look down the left-most "Capo" column to the number 2. If you read the next two columns on the same row, they mean that when you place a capo behind the second fret and play in the guitar key of C, the real "concert" key is D, because the capo has raised the tone by two frets (which equals two half steps). In this case, a guitar C chord really sounds like a D chord on the piano. Likewise, moving farther over on the same capo-2 row, a guitar G chord really sounds like an A chord on the piano.
The second table contains the same information, but it is organized differently (Settings Organized by Real Key). Here the left-most column shows the real key. So if you want to play in the key of G#, you can place the capo at 4 and play the guitar in the key of E. In this case, the E chord on the guitar will sound the same as a G# chord on the piano. Looking across the same row, it shows that you can place the capo behind fret one and play in the guitar key of G in order to sound the real key of G# (which is also Ab).
Acoustic Music Webtools