I agree that the fact that both Chell and GLaDOS are female is irrelevant to gameplay. I think the creators of Portal wanted to make a puzzle game, so they came up with the different levels and challenges, and then focused on the skin of the game (i.e. what the main characters should look like). In contrast, I think the creators of Tomb Raider knew from the beginning that they wanted to feature a female main character in their game. So, they fine-tuned Lara Croft’s appearance, and then developed the gameplay around her. With this begin said, I think the two games are slightly different since Portal is more of a first-person-shooter game and Tomb Raider is more of a third-person adventure game. So, obviously the point of view is going to differ accordingly and the player will be able to see more or less of their character.
As far as GLaDOS, I think the game designers purposefully made her a female voice. In general, I think a woman’s voice is more trustworthy and comforting. This is perfect for Portal since for a large portion of the game you must rely on a mysterious voice to navigate the gameworld. Also, I think having it be a feminine voice allows GLaDOS to have more attitude while remaining ironically funny in some of her statements (especially closer to the end of the game)–if a male voice said some of the things GLaDOS says, players may think it is rude or offensive (sorry, guys).
Lastly, in regards to the ‘Remember, the Aperture Science Bring-Your-Daughter-To-Work Day’ statement, this refers to the background story of Portal, which we don’t find out until Portal 2. Apprarently, the reason why the testing facility is so empty is because GLaDOS went beserk and released a poisonous gas on attendees of Bring-Your-Daughter-To-Work Day.