To be exact, buffs stacking work the following ways:

Any buff from two DIFFERENT ninja, even if they seem to be the same, stack

Any buff from two of the SAME ninja does not stack, except shields. While this usually applies to 3v3 team battle where there are the same cross-team buff on field (pains, for example) since you can't have two of the same ninja on your field at once, there are corner cases in 1v1 such as Yagura copy.

And since you mentioned Gaara specifically, there can only be ONE barrier, which is how Gaara "buffs", it's a barrier. So Garra+Onoki won't work, because both "buff" through barrier, which you can only have one of(in fact, you might have neither if your OPPONENT is the one with the barrier)