[LudiusMaximus]

Instead of

local BlizzardGameTooltipOnUpdateRetention = GameTooltip.OnUpdate

you would have to do

local BlizzardGameTooltipOnUpdateRetention = GameTooltip:GetScript("OnUpdate")

But what you are doing then, overriding the original OnUpdate script is an insecure hook.

Is it important that your CustomToolTipOnUpdate() gets called before BlizzardGameTooltipOnUpdateRetention()? If not, just do a HookScript:

GameTooltip:HookScript("OnUpdate", function(self)
   -- Your stuff here. E.g.:
   for i = 1, self:NumLines(), 1 do
     local line = _G[self:GetName().."TextLeft"..i]:GetText()
     print (i, line)
   end   
end)