That's odd. As said here it shouldn't be:
http://www.gog.com/en/news/gog_com_holiday_sale_begins

But you're right as when I go here:
http://www.gog.com/en/gamecard/gothic_3/
It does say 4.99.

Can't say for G2 as I've already purchased it there so it doesn't list the price for me on:
http://www.gog.com/en/gamecard/gothic_2_gold_edition