This was sort of answered in this thread
https://forum.thegamecreators.com/thread/222424
Basically, you know the height of the text so you simply offset the text by its height if you want it bottom justified
SetTextPosition(id,GetTextPosition(id),GetTextPosition(id)-GetTextTotalheight(id))
If you want it centered - use half the height
SetTextPosition(id,GetTextPosition(id),GetTextPosition(id)-GetTextTotalheight(id)/2)
Its only one line ad these only work if there is no angle to the text.
If there is an angle then use the function provided by the thread above. (The function listed by IronManhood is perfect for centering text but chnage the 0.5 to 1.0 and it bottom justifies it.
A function in AppGameKit to do this would be nice but not absolutely needed.