Craft a musical composition that adheres to the specified musical theme and letter-based structure.
X:1
L:1/8
Q:1/4=120
M:4/4
K:G
['G/A/B/c/ dB gBdB', 'gage abaf']  
X:1
L:1/8
Q:1/4=120
M:4/4
K:G
 B | G/A/B/c/ dB gBdB | gBdB aAAB | G/A/B/c/ dB gBdB | cAdc BGG :: f | gage abaf | gage beef | 
 gage abaf | gdec BGG :|  