Do we get more deals every day? Or does GOG simply reshuffle games on the front page?

New games are discounted (for a set period); sometimes a game will be discounted a second time (but typically for a different amount, e.g., say, first for 75%, then for 33% — or vice versa).

Usually, the discount you see when the promo started, will not change.
GOG did have several bugs before and changed discounts in various promo.
But usually, they keep the discount to the end of the promo.

GOG add some games to the promo today. Usually, they will keep it to the end of the promo.

That said, sometimes it takes up to about 24 hours or less to list all the games sometimes. After that it seems to not change very often, unless they add a few in the last 24 hours.

But I could be wrong, so subject to change. ;)

Yeah, I wasn't very clear — a popular game in a long sale period (not necessarily this Black Friday, but for a seasonal sale period, when there are days of specials bundled together) may have a second discount period. Or at least, I have noticed it before in previous sales …

Some times with the weekly sales, they would add games on Fridays....