Posted by David Hamburger
Excerpted from The Acoustic Guitar Method
The G7 chord, as you might guess, is closely related to G. Two of the three fingers for this chord are the same as for G—only the high string changes, from the third fret with your pinky to the first fret with your index finger.
The G7 feels and looks a little like a wider, more spread-out C chord. You can use the same trick of first switching just your middle and ring fingers between G7 and C before switching between the full three-finger version of each chord. Then spend a little time switching between C and G7 just using a basic strum, as in Example 1.
It’s a good idea to be able to switch between G and G7 as well. In Example 2, practice getting your pinky out of the way and getting your index finger in there on the first fret.