Gifting wouldn't really lower it much, because the score is calculated using the integral of the graph line for each scoring portion (military, population, tech) etc. This means the total area under each curve, then modified based on how many turns it took, what type of victory is achieved, difficulty level, and galaxy size.
So I guess do a cake walk tiny galaxy, do jack the whole time, tech victory gives lowest modifier and rare habitable planets and see what you get. You can probably get like a 100 score or something haha.
EDIT: Oh, and take an insane amount of turns to do it too. like hundreds.