GI - Hee
A "G" before the letter I in Spanish sounds like an English H. So GI becomes "HEE".
For example you would pronounce the word girar (to turn) as "hee-rahr".
The letters GI are pronounces HEE
To remember this we use the following mnemonic:
The GI (as in GI Joe) was a real he-man (GI-hee).