Son Goku has been seen to wear numerous outfits, but has mostly been associated to wear several variations of the Turtle School Uniform. His family gi is usually depicted as blue gi that has no kanji, as he is seen to revert to this color by the end of the ten year gap following the defeat of Majin Boo.

Son Family Gi Variations:

This is Son Goku's Gi when he is first introduced in the series.
He wears this gi from chapter 1 to chapter 20 of the manga series
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This is Son Goku's gi at the end of the canon series. He wears this
at the Tenkaichi Budokai when he meets the character Oob
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This variation of the Son Family gi is used in the anime only spin-off
Dragon Ball GT. The only major difference from the gi seen at the
end of the canon series is that Son Goku's pants have gone from
green to yellow.

Turtle School Gi Variations:

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This gi is the original Turtle School gi that Son Goku was given.
Unlike his family training gi, the Turtle School gi is depicted as
orange or red, and has a "kanji" (symbol) on both the front and
back meaning "turtle".
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This is the second variation of the Turtle School gi that Son
Goku wears. The only major difference from the original gi
is the blue undershirt and the change from shoes to boots.
This gi appears at the very end of Dragon Ball, and into
Dragon Ball Z. This version of the Turtle School Gi goes
through several kanji changes. The first change is when Son Goku wears the
Kaio symbol on the back, keeping the original Turtle kanji on
the front. He later changes the front Kanji to his own symbol, meaning "Enlightenment".
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This is the third and final version of the Turtle School Gi that Son Goku wears.
This version disposes of the Kanji, and changes the tie belt to a sash.
This change was made after Son Goku achieved Super Saiyan, first appearing
in his battle against the Artificial Humans.