who are the main actors in it?
Blake Lively plays Serena, Leighton Meester plays Blair, Chace Crawford plays Nate, Penn Badgley plays Dan, Ed Westwick plays Chuck, Taylor Momsen plays Jenny and Jessica Szohr plays Vanessa.
Gossip Girl (novel series)
 In the show, Blake Lively plays Serena, Leighton Meester plays Blair, Chace Crawford plays Nate, Penn Badgley plays Dan, Ed Westwick plays Chuck, Taylor Momsen plays Jenny and Jessica Szohr plays Vanessa. 
who ends up being the father of blair's baby Louis Grimaldi. the character mentioned above is from which series? Gossip Girl.
wiki:17831336
Gossip Girl (novel series) [SEP] Television adaptation
The television adaptation of the novel series, also titled "Gossip Girl", was picked up by The CW. Josh Schwartz, the creator of "The O.C.", is executive producer for the project. In the show, Blake Lively plays Serena, Leighton Meester plays Blair, Chace Crawford plays Nate, Penn Badgley plays Dan, Ed Westwick plays Chuck, Taylor Momsen plays Jenny and Jessica Szohr plays Vanessa. The show is loosely based on the books and does not follow the same story line. Some key characters from the books, such as Aaron Rose or the Lord, are introduced into the show with different storylines, and some characters undergo changes to their personality and characteristics.