The melded card will probably be your commander. If both Gisela or Bruna are your commander firstly of the sport, once they meld collectively Brisela will nonetheless be thought-about your commander for all functions.
When and if Brisela leaves the battlefield solely the unique commander half will comply with the commander guidelines for returning to command zone, the opposite half will behave as some other card would. This is roofed within the rulings for each Bruna and Gisela:
In a Commander recreation, your commander could also be Bruna, the Fading Light or Gisela, the Broken Blade, and the opposite could also be in your deck. If they meld into Brisela, Voice of Nightmares, Brisela may even be your commander; but when Brisela leaves the battlefield, solely the cardboard chosen as your commander firstly of the sport could also be put into the command zone.