In Huxley's "Flame Trees of Thinka," she states that there is no word in Gikuyu for "thank you." I'm assuming that's false - can someone confirm?

You are right! She is Wrong.

"Thengiû"

"Nî Ngatho"

These are Gîkûyû words meaning "Thank you"...

Psalm 75.1 (in Gîkûyû)
"NGAI, Nî ngatho twagûcokeria; Nîtwagîcokia Ngatho, nî tondû rîîtwa rîaku rîtûikaraga ikuhî; andû magakîmemerekia maûndû ma magegania maria wîkîte."

Psalm 75.1 (KJV)
"Unto thee, O God, do we give thanks, unto thee do we give thanks: for that thy name is near thy wondrous works declare."