The origins of the "G-Major" meme appear to have first started on YouTube and have remained quite popular to his day, due to its "scary" and "creepy" sound effects. The earliest know video that utilizes G-major is PanicBomber's Scaryroll and the view count has nearly reached one million views.
As for the relationship between G-major and Giygas, I assume that both subjects fascinate fans due to its "Horror" elements that is entertaining for some people.
Citations:
KnowYourMeme.com
In G Major was created in 1975.
A G major is used in music. A G major is usually played on instruments. Some examples of instruments that plays G major include guitar, violin, and piano.
The 'G' Major chord is 'G','D','B'
Tuba major scales refer to circle of fourths where G major is concert G.
G major, C major, D major.
G Major (surprise, surprise)
G
E Minor is the relative minor to G Major.
G Major
The notes of the G Major chord are G, B and D.
G Major has just 1 sharp and it is F#
E Minor is the relative minor to G Major.