There isn't a single perfect way to write "Garrett" in Japanese, as there's no direct equivalent. The best approach is to use a phonetic approximation using katakana, which is generally used for foreign names. The most common and natural-sounding transliteration would be:
ガレット (Garetto)
This is a straightforward phonetic rendering. Other possibilities exist, but they might sound less natural to a Japanese speaker. Using hiragana is generally avoided for names.