Gilder Bairn

• WWizards of the Coast2008-08-01
  Official Ruling
  If the permanent is already untapped, you can’t activate its ability. That’s because you can’t pay the “Untap this permanent” cost.
• WWizards of the Coast2008-08-01
  Official Ruling
  Gilder Bairn’s ability will replicate each kind of counter on the targeted permanent, not just +1/+1 or -1/-1 counters.
• WWizards of the Coast2008-08-01
  Official Ruling
  When you activate an ability, you untap the creature with that ability as a cost. The untap can’t be responded to. (The actual ability can be responded to, of course.)
• WWizards of the Coast2008-08-01
  Official Ruling
  The “summoning sickness” rule applies to . If a creature with an ability hasn’t been under your control since your most recent turn began, you can’t activate that ability. Ignore this rule if the creature also has haste.

Rulings via Scryfall