Develop a musical piece using the given theme and incorporating the letter-based structure provided.
X:1
L:1/8
M:6/8
K:G
['GBB dBB GBB g2 B FAA dAA', 'g3 (f/g/a)f (f/g/a)d (f/g/a) a2 d bag']  
X:1
L:1/8
M:6/8
K:G
|: GBB dBB | GBB g2 B | FAA dAA | FAA f2 A | GBB dBd | gdg bgb | afd (e/f/g)B | Agf g3 :: 
 (f/g/a)f (f/g/a)d | (f/g/a) a2 d | bag fdf | eA^c d2 B | cee caa | Bdd Bgg | bgd (e/f/g)B | 
 Agf g3 :|  