The phrase 'God bless you' translates into Hawaiian as 'Ke Akua Hoomaikai Oe'. If you were saying this as a farewell, you could also say 'Ke Akua pu a hui hou' which means 'God bless you and goodbye.'
'Akua' is the Hawaiian word for 'God'. The direct translations for 'Bless' and 'You' are 'Ho'omaika'i' and 'oe' respectively. Bear in mind direct translations don't always give the full effect of the phrase, and Hawaiian has a smaller alphabet than English, make some words look unusual or similar to others.
Other Spiritual Phrases in Hawaiian
God is love - Aloha ke akua (Aloha is 'love' but can also be used as a general greeting or farewell in Hawaii).
Family god or spirit - 'Aumakua
Thanks be to God for this day - Mahalo E Ke Akua No Keia La
May blessings be ever with you - E pili mau na pomaika`i ia `oe
Happy Christmas - Mele kalikimaka
God's house of grace - Ke aloha o ke Hale Akua
The survival of the native Hawaiian language is a constant concern in Hawaii, and can provoke lively debate, as we see in this PBS discussion show: