So, to basically put it in a simple way, Sword Art Online is about this MMORPG virtual reality game and the players can log in and play and interact with other players and it is like the virtual game became reality. Soon, they start to realize that they cannot log out. The only way they can log out is if they win and beat all 100 levels. If in any circumstances you die in the game, if someone in the real world take off your head gear, etc. then you will die in real life. The main characters of the game are Kirito and Asuna.
For those of you who have never seen Sword Art Online or is wanting to see it but is still deciding, here is a video of the Sword Art Online trailer:
Now, for Gun Gale Online! I have never seen Gun Gale Online and I really don't know much about it either. But, I am planning to watch it as soon as I can because I am really interested in seeing since I really enjoyed watching SAO. At first, I didn't know that Gun Gale Online would be season 2 of SAO because you would think that they would keep the name SAO and just call it "SAO 2" or "Sword Art Online" and just say that it is season 2. So, I really didn't get it but, then I thought about it and it started to make sense to me that they just changed it to make it set the theme of the type of game genre it is. "Sword Art Online" had to do with fighting and battling with swords. And now, Gun Gale Online have to do with shooting.
Now, I really cannot say much about this anime because I have never seen it or read any of the light novels so, for you guys who would like more details about this anime, I suggest that you visit the Gun Gale Online wikia (CLICK HERE).
I've been trying to find a cool and decent trailer of Gun Gale Online for you guys so, here is the best one I can find on YouTube:
I hope you guys will enjoy Gun Gale Online!