The G major seventh chord is known as the G7 guitar chord (pronounced G seventh). G7 uses all six strings, and is commonly used as a transition chord from the G chord.
The chord diagram on the left is for right handed players, and, the diagram on the right is for left handed guitarists.
An example of how a transition chord would be used is: G, G, G, G7, C instead of G, G, G, G, C. Using a G7 instead of a G for the last chord in a measure adds a bit of variety in the chord progression.
Of course G7 isn't only used as a transition chord.
As you can see in the chord diagram, G7 is played with the first finger fretting the first string at the first fret instead of the fourth finger at the third fret.
Switch back and forth between G and the G7 chord by exchanging your first and fourth finger positions.
I hope you learned to play the G chord the way I showed you, using your second, third, and fourth fingers, instead of using your first three fingers! If you didn't, it's time to learn the G chord again the way I showed you(it will make a lot of chord changes easier).
Only two more easy guitar chords to learn, the C7 and D7.