Develop a musical piece using the given theme and incorporating the letter-based structure provided.
X:1
L:1/8
M:2/4
K:G
['BGAG FAAF GABA']  
X:1
L:1/8
M:2/4
K:G
 D | G>DGA | B3 G | c>BB/A/G | A2 z2 | B>AGB | BGAG | FAAF | GABA | B2 z2 | B>AGA | BGAG | FAAF | 
 GABA | G2 z |]  