If you build a drop-3 voicing from a chord in root position, you end up with a chord in first inversion. This is because to build a drop-3 voicing you take a close voicing and lower the third-highest note by an octave, making that note (which is the 3rd of the chord) the lowest note of the chord. To understand why the voicing in question is a drop-3 voicing we need to consider the inversions of drop-3 voicings.
Let's build a drop-3 voicing on the guitar, but let's do it for CMaj7 instead for reasons that will become apparent.
Start with a close voicing of CMaj7:
Now to construct a drop-3 voicing, lower the third note from the top by one octave:
This turns out to be an awkward way to finger this chord (for example, try converting this CMaj7 to a C7). Let's move the C on the 5th string to 4th string; this is the same set of notes, only a different fingering:
This is a drop-3 voicing for a CMaj7, but since we placed the E in the bass, it is in first inversion. The spelling is E C G B, and we can find the other inversions of this voicing on the neck by moving each note along its string to the nearest neighbor in the chord. To find the root-position voicing, move the E down to C, the C down to B, the G down to E, and the B down to G:
This is a root-position CMaj7 with a drop-3 voicing, and the same voicing used in the GMaj7 from the question. The third inversion can be found by sliding along the strings again, C down to B, B down to G, E down to C, and G down to E:
This is a drop-3 voicing of CMaj7 in third inversion. Sliding down again, B to G, G to E, C to B, and E to C:
This is a drop-3 voicing of CMaj7 in second inversion. And those are all of the fingerings for the inversions of drop-3 voicings of Maj7 chords on the 6-4-3-2 string set.
Another way to think about this would be to start with a close-voiced Maj7 chord in third inversion. This is all but unplayable on a guitar in standard tuning, but it can be played using an open string for AMaj7; that would be spelled G# A C# E:
Drop the third note from the top, which is the A, an octave to get a drop-3 voicing:
This is another awkward fingering. Keeping the notes the same, but rearranging the fingering gives the classic root-position drop-3 voicing for a Maj7 chord: