The letter g has a different pronunciation when combined with all vowels. When it is accompanied by the vowels "a, o, u" it is pronounced as in the word "go" in English. When it is accompanied by the vowels "e, i" it is pronounced as an "h" in the word "hope". In order for it to sound as a "g" as in the word "go" with the vowels "e, i", we must add a "u" gu . Let's look at a few examples to understand better.

g
Mi amiga tiene dos gatos.
     My friend has two cats.
Tú eres inteligente.
     You are intelligent.
Nuestro colegio.
     Our school.
Anotó gol.
     He scored a goal.
El gusano gordo.
     The fat worm.

gu (with vowels "e, i" only)
Tu juguete.
     Your toy.
Tocar la guitarra.
     To play the guitar.
La manguera es verde.
     The hose is green.