In addition to that, it might be "equal" to the house in terms of being good enough to count as GLA, but still not as good in terms of functionality or even finish.
Think of it this way... are the kitchen and the bedroom "equal?" So, even if it is nicely finished and could count as GLA, it may still not be the same, when compared to other property with the same GLA or in the cost approach.
I've found that, in some cases, it should not be counted as GLA, in other cases it could, and would be worth just as much as the rest of the living space, and in still other cases, it is GLA, but not worth as much as the rest of the living space. This is really the appraisers call, IMHO.
The best bet, in most cases, is to consider this space separately in the cost approach. The best bet, in most cases, is to find comparable sales that also have garage space finished out for use in the sales comparison approach.