Hi Bob, The note at the first fret of the 1st (high E) string is an F note. The G7 chord (all 7th chords actually) use the 7th degree of the scale...in this case the F note at fret 1 of the high E string in order to form the 7th chord. A G chord is made up of three notes G, B and D. To make a 7th chord the F note (7th) must be added. The G7 chord contains the G, B, D and F notes. Some of these notes repeat on the open G, B and D strings when we form the standard G chord shape. I hope this helps to answer your question. Hope you enjoy your guitar journey!

Jim