The key: what “the song is in G” means
A key is a family of chords around one resting point. Playing a G grip does not yet mean the song is in G.
Adrian3 minupdated
The letter next to the title does not mean you play that one grip from start to finish. A key is a family of chords that circle one resting point. In C that point is C. In G it is G. Playing a G in a song in C does not change the key to G.
Hear the rest: C, F, G, and back to C
Play one bar each: C, F, G. After G, stop your hand. The sound is fine, but the phrase has not sat down yet. Play C. That is the practical sense of the key of C: C closes the loop. Ending on G leaves a question.
If F slows the change, prepare the grip earlier. Here you are listening for direction, not finger speed.
The first and last chord are a clue, not a verdict
In a simple arrangement the first or last chord often matches the key. Not always. In Guitarmatic the surest answer is the header on the song page. The ear helps you understand that entry. It does not replace it.
Jingle Bells is in C and uses C, F and G. The last bar lands on C. Here the clue and the card agree.
Tryumfy Króla Niebieskiego is in G and uses G, C and D. The card says G. The first and last bars are C. If you took only the outer chords, you would name the wrong key. G is the resting point of this carol, even when the phrase starts and ends on C.
Three pillars by ear, without an exam
In Ear: chords, level 2 you hear C, F and G. First look for C as the place where the loop can end. Then tell F from G. A miss does not score talent. Go back to C-F-G-C and compare again.
Play C, F, G and return to C
After G, stop your hand. Then add C. The key of C is that loop, not the C grip on its own.
Measure: you hear that an ending on C sits, and an ending on G leaves a question.