Obtained from Decisive Battle of the Valkyries event.
Gondul's Main Skill can be modified by equipping special Equipment Subs from her Event:
As noted, it's an unconditional 15% Skill Boost to any card. However, if you sub Gondul along with either Olrun, Randgris, or Geirolul, this becomes a massive 45% Skill Boost instead!
Needless to say, this makes Gondul an excellent sub for Slash, Hit, or Stab cards (to take advantage of the other subs' skills), but the boost is also big enough to consider using on other cards.
Note that if you're using the appropriate skill type Valkyrie as a sub with Gondul, the skill boost is very competitive with using 3x35% subs:
Gondul + Valkyrie + 35% sub
1.45*1.25*1.35 = 2.446
3x 35% subs
1.35^3 = 2.46
This makes Gondul+Valkyrie an extremely affordable option against 3x35% subs.
In Norse mythology, Göndul (Old Norse "wand-wielder") is a valkyrie. In addition, Göndul appears within the valkyrie list in the Poetic Edda poem Völuspá, in both of the two Nafnaþulur lists found in the Prose Edda, and among the valkyries listed in Darraðarljóð.