I want to gift a gog game to a friend who loved capitalism 2, may you please suggest me some similar games available on GOG's catalogue?
Thank you very much :)

You could consider Capitalism Plus (obviously), if your friend hasn't played it yet. It's not as pretty as Cap2, but it is more complex.
Other than that, unfortunately, there are hardly any games that are so in-depth. On GOG, these are the only two "managerial" style games.

You might want to take a look on Patrician 3, Port Royale 2, and The Guild Gold. They are all heavily economy-based, in different ways.

They are, but they are nowhere near the Capitalism games in compleyity.
(However, it that's not a problem, you should indeed check them out; Patrician 3 and Port Royale 2 in particular put emphasis on management.)

Thank you very much for your help, probably he will like Port Royale 2

I second the recommendation of Patrician 3, and I don't really think it's any less complex than Capitalism 2. It is, IMO, a better game as well (though to be fair, I also consider Cap Plus to be better than Cap 2).

Always this site provided many knowledgeable links here so I visit on this site time to time...
Extreme Whitener

Agreed, P3 is a fine game. In reality though it is P2 patched, repackaged, and resold to a waiting crowd as a totally new game! Great game none-the-less.

Don't forget the fact that it includes the expansion that was only released for the German version of P2. The reason they did it like this in the first place was probably that P2 was never a big success in the non-German markets, so there was no point in releasing the expansion for the few existing P2-customers.