I might be missing something, but since both of them are legendaries that first appeared in the Core Set, and are still in rotation, i would also expect both of them to be craftable within the Legacy Set.
But only Gnomelia is craftable, meanwhile Deathwing the Destroyer, Onyxia the Broodmother and Malygos the Spellweaver are not.
The “bug” being that Gnomelia and Co are already available in the legacy set. Proof being that the Legacy version of Gnomelia doesn’t exist on the website library.
Cards that are introduced BY the core set are only supposed to enter legacy after they’ve been rotated out.
All the dragons you’ve mentionned never left core set since they’ve been added to the game. Same as Resistance Aura and Crusader’s aura.
Ysera the Dreamer and Alexstrasza Life-Binder got introduced by and rotated out of core set and are now in the legacy set.
Crimson Clergy got introduced by the core set, got rotated out then rotated back in, which makes it one of the few exceptions.
Interesting… Then I guess it’d be more trouble than worth to fix it, since weird things might happen if someone crafts Gnomelia and she gets removed from Legacy.
Thanks for the insight