GilderBairn and Blind Obed.

[Mattao19]

 {Gilder Bairn} {Blind Obedience}

My opponent controls a BO and I play GB. Can I untap GB as soon as it hits the field? Or does sum. sickness still apply?

[Lyriczulu]

302.6. A creature's activated ability with the tap symbol or the untap symbol in its activation cost can't be activated unless the creature has been under its controller's control continuously since his or her most recent turn began. A creature can't attack unless it has been under its controller's control continuously since his or her most recent turn began. This rule is informally called the "summoning sickness" rule. Ignore this rule for creatures with haste

So yes, unfortunately summoning sickness applies to untapping as well.

[Mattao19]

Thanks bud!

[Lyriczulu]

No problem