aim or tacticsf or GSFC?

Tactics will check proc in every 10 second and it will check while it is toggled on regardless if you are in combat or not.

Aim will check it whenever you use power so you can be sure that it will always be used in combat and if your aim has a recharge less than 10 second (and assuming that you are using aim as soon as it is available) it will be better place for GSFC 6 slotted.

Aim is on a base 90 second recharge, so it can't be cut to less than 18 seconds even at the recharge cap.

That being said, you'll probably get more use out of aim than tactics. Aim is also a decent damage buff in addition to the to-hit buff. And although the GSFC proc will go off less often, you'll pretty much guarantee that every time it goes off you'll get some use out of it, since one rarely uses aim and then doesn't use any attacks. In tactics it will just go off randomly, and it's hard to tell that it's gone off so that you can fire attacks as well.