Let’s look at them individually first.
Bruna, the Fading Light costs 7 mana to cast, but she’s a 5/7 with flying and vigilance, and has a really powerful ability. When Bruna enters the battlefield, you get to return a target Angel or Human creature from the graveyard to the battlefield. That is just as good as it sounds. This means you can bring Gisela, the Broken Blade back to the battlefield from the graveyard very easily. So, now, we get to see what these two get to meld into.
Obviously, on paper, this trio looks extremely good. Gisela, the Broken Blade is probably good enough on her own to see play. Bruna, the Fading Light is well worth the 7 mana, too. The melding concept is quite fascinating. So, just how often would these two actually become Brisela?
Brisela, Voice of Nightmares in Orzhov Control
As it turned out, not only were Gisela and Bruna fairly playable in a mid-range White/Black Control deck, but being able to become Brisela were pretty much worth playing both of them. Gisela was fairly easy to remove with her 3 toughness, but because you could just cast Bruna to bring her back, that wasn't really an issue.
The deck archetype, often called Orzhov Control, but perhaps more appropriately called B/W Angel Control, saw lots of success playing two copies of Gisela and one to two copies of Bruna. It also played two copies of Archangel Avacyn and the very powerful planeswalkers Gideon, Ally of Zendikar and Liliana, the Last Hope. While the deck certainly didn't need to meld a Brisela to win, it certainly didn't hurt when it happened. The deck also played Thalia's Lancers, which made it easy to search out whatever Legendary Creature you wanted.
Later, some Mono-White Eldrazi decks ran one copy each of Bruna and Gisela. There would also be Boros Angels and Bant Angels decks that would incorporate some Humans in the mix and actually run 3 or even 4 copies of each Bruna and Gisela.
Gisela, the Broken Blade would see play on her own in winning Azorius Aggro decks. And in some very competitive White Weenie lists, you'd sometimes find a copy of each Bruna and Gisela. Melding into Brisela gave typically small creature decks an explosive win condition. Throughout their time in Standard, Gisela and Bruna would pop up in one-ofs in other aggro decks, too, just because Brisela was just so difficult to answer.
After rotating from Standard, neither Gisela nor Bruna has seen any competitive play. However, their presence in EDH is another story!
Bruna, the Fading Light in EDH / Commander
As a Commander, Bruna is really nothing special. But in the 99 of other commanders, that's a different story. She's become a staple of Kaalia the Vast decks that focus on Angels. Really, she's good in any deck where you want to reanimate any Human or Angel for 7 mana. Plus, she's a 5/7 flyer with vigilance, so she's a pretty formidable body on the battlefield, too!
Gisela, the Broken Blade in EDH / Commander
Like Bruna, Gisela is an auto-include in many Angel Tribal decks, especially Kaalia of the Vast. According to EDHrec, Gisela sees play alongside Bruna in over 85 percent of possible decks. Their meld combo is just that good, even in Commander!
Value-wise, Bruna and Gisela are quite different in terms of price. Bruna, the Fading Light is a dollar rare with foils in the $5 range. However, Gisela is north of $6 with foils surpassing $15. This wide disparity in value stems from the fact that players bought far more copies of Gisela who saw more competitive play in Aggro decks than Bruna. Gisela was also a $10 card for a long time. So, while they almost exclusively see play together now, there are just many more copies of Bruna still available on the market.
Bruna, the Fading Light and Gisela, the Broken Blade are definitely cards you want to have around if you plan on playing any Angel tribal decks. They may not be seeing competitive play outside Commander anymore, but they had a good Standard run and their meld into Brisela is still a frightening thing to stare down whenever it hits the table.