There's actually an optional rule for Adepts to take a Geas associated with a specific adept power. Page 31 has a huge "sidebar" and the Adepts and Geasa is at the end of the first column. Basically the optional rule lets you take a geas on a power to get a discounted price for the power (in that case it isn't also a negative quality)
If you are using the optional rule, then the geas only applies to that single power. Otherwise it applies to all of the character's magic.
If you are taking a (gesture) Geas as a negative quality, then it is going to be the same gesture or type of gesture for any and all magic that the character performs. It could be like bending from Avatar, the hand signs from Naruto, and I've even seen someone use "smoking" as a geas (basically the combination of having a lit cigarette, and the motions involved with that).
Generally speaking, whatever the Geas, it needs to be done when you perform the action, for extremely long powers your GM might require you to "renew" your geas, but you definitely don't have to be constantly performing the gestures.