[Fizzlemizz]

That's because the widget type you're "getting" is a Button. Buttons don't have a SetText or Setfont method by default but in some case they are "given" a SetText method to update a FontString they have also been given.

local Button = CreateFrame("Button", "SomeButton", UIParent)
Button.Text = Button:CreateFontString(GameFontNormal)
Button.Text:SetPoint("CENTER")
function Button:SetText(text)
    self.Text:SetText(text)
end

You can

Button:SetText("Something")

but you can't

Button:SetFont(SomeFont)

but you could

Button.Text:SetFont(SomeFont)

In this case, you need to find the FontString "sub-widget" of GTxtframe you want to effect (like .GreetingText, it may be something other than .Text).