I've noticed that all Tropico games are on sale as a "Daily Deal", with banner and all:

https://www.gog.com/promo/20180219_daily_deal_tropico

Are daily deals something new or it's just a special deal like some other sales that pop up from time to time?

Not entirely sure but sometimes GOG has special promos along with the weekly sale. So this could be a special promo.

True, but they never had been called 'Daily Deals' and lasted only 24h.

The Tropico deal is over now, and there's a new Daily Deal on, so there may be a new 'Daily Deal' every day from now on... Though the lack of an official announcement probably means that GOG is testing the waters here before committing to a new fixed sales format.

Yes you are right maybe this is something new. More choice is always nice, let's see if the daily sales are here to say.

Sure they do, they have daily sales, weekly sales, monthly sales, quarterly sales, special sales, time limited sales, in fact there isn't a second goes by without at least 3 or 4 separate sales ongoing. Trafalgar had fewer sails in one place at one time than here.

It seems to me that it would be great if it was so. But experience shows that there is no such thing.

Today's daily deal is an anomaly, hua hua hua

I chuckled. : )

I think i already like these daily deals; not only they offer more chances to buy a game that i'd otherwise have waited for a big sale (maybe because i'm just only mildly interested/not sure if i like it/just for the history/for any other reason), but GOG will generate more revenue as well. Having said that, be careful of impulse buying and don't let those backlogs get huge because they'll eat you in the end!