Hello,
The GIRM, in several instances, says something like “the priest, facing the people…” (for example, see. #124, 146, etc). If the GIRM is specifically instructing the priest to face the people at these times, the conclusion is that he is facing the other way the rest of the time he is at the altar. Does that make sense?
In some churches or at some altars, the only direction the priest can face, when he is at the altar, is toward the altar. For instance, last week Pope Francis celebrated Mass facing the altar because that’s the only way he could offer Mass at that altar. The GIRM recognizes this, at the very least.
I think it is also reasonable to conclude that since the GIRM, in fact, does *not *instruct all priests to celebrate “facing the people” all the time, it gives them the option to face either way–even if the altar is positioned in such a way that facing the people is possible.
I hope this helps.
Dan