I remember I bought For The Glory for like 5 USD and Europa Universalis 3 Complete for no more than 7 USD during a promo that happened like in last November. In this New year promo they cost, respectively, 9.98 and 13.98 USD.

I bought during these holiday sales in the end of December the game Star Empire V, for 7,98 (50% discount) and then read in the comments ( I was careless enough to not read them before) people commenting that they got it for 2.50 USD in a promo in the past.

This makes me a bit hesitant to buy games now since they could very well enter a promo in the next year for like over 75% off, and I already have lots of games to play so don´t really need to buy something right now.

So, do they make sales frequently, like every weekend?

GamersGate holds week long sales almost every week.

There's week-long sales (game of the week), and there's weekend sales.

Gamersgate has sales EVERY week.

They also have loyalty discounts too. The more you buy from them, the higher the discount you get for these loyalty sales.