@jp523  
Sort of like the exact opposite to Phage

When Gabby enters the battlefield all players win.

@Background Pony #D0B9  

 
Good point. Most cards like this trigger before attackers are declared.
 
“At the beginning of combat, if a creature you control has first strike and it and Gabby are able to attack or block this turn, you may have Gabby gain first strike and both those creatures attack or block this turn if able. The same is true for etc.”

My first thought was: “Haste and vigilance make no sense in that situation.”  
Might also be a bit too cheap even with the downside. Probably better with 2W or WW instead of 1W.
 
Still damn adorable though ;)

@Phil Srobeighn  
Nine times out of ten, that’s my creative process. “Creative synthesis” is just a fancy way of saying “borrow from enough sources and it’ll look original.” ;)

@FanOfMostEverything  
When I searched for how to order the abilities and saw Odric, I knew that I would not be anywhere close to your Gabby. You come up with cards that are original, whereas I am posting this as a reaction to “This character reminds me of this established ability in Magic.”

It’d be a bit redundant, but I’d love to see Odric ride into battle astride Gabby.

@Phil Srobeighn  
The cost is probably fine, compare it to any of the w1 slivers out there and it doesn’t seem anywhere near OP.
 
Yes, I used to run sliver white-weenie, there was tears.

@Keith Mowz  
Silverspark is right that you can have as many in the deck as the format you’re playing allows, usually four. As far as Power Level goes, when I was looking up the right word order for her I noticed how similar she was to Odric, Lunarch Marshal and finally had her at two under his cost due to the fact that you can’t even use her as an attacker or blocker alone, let alone start to clone abilities. If I had kept her at a Knight as original (because she’s a Crusader, get it?) I may have gone with WW because I love WW knights.
 
@Silverspark  
Technically nowadays you get to choose which one to keep and the rest are immediately sent to the graveyard. In case you play four Legends in a deck and want to get rid of a pacifism/load of negative counters/nasty Vampire bite.
 
@Damaged  
You’re right. Spur of the moment response should have been better reviewed.  
Crusader’s Mark (3)  
Artifact – Equipment  
Equip 2  
When you equip ~, choose Horse, Unicorn, or Pegasus. If Equipped creature is not a Horse, Unicorn, or Pegasus, it is the chosen type in addition to its other types and gains flanking.

@Phil Srobeighn  
Would more likely become a “gains” effect rather than overwriting their type. Most such things (that overwrote a creature type of another card) have been erratad.

@Keith Mowz  
You can only have one legendary with the same name in play, but you can have up to 4 in a deck. When you play one with a copy already in play, it is immediately destroyed.

You can only have one copy of a legendary in a deck, yes?
 
It’s been too long since I played because that ability cloning thing seems potentially REALLY strong for a 2-mana card. Like, not even WW, 1W?
 
Really the main thing that I would say makes it fair is how the dynamic of requiring her to block/attack along with whatever it is giving her the bonus traits makes it more fair. Yeah, you can give her first strike and death touch, but that requires you to also attack/defend with both the first strike and deathtouch characters, which will potentially get them killed.

@Damaged  
Crusader’s Mark (3)  
Artifact - Equipment  
Equip 2  
When you equip ~, choose Horse, Unicorn, or Pegasus. If Equipped creature is not a Horse, Unicorn, or Pegasus, it becomes the chosen type and gains flanking.

What? Gabby can’t get Flanking or Horsemanship? flips table

Decent size-to-cost ratio, very on color ability for white. Not worth making an EDH deck around, but could be a great general in one. I like it.