In this context he is talking about the Oral and Written Law. You need to be careful about this because nowadays many people think any ideas that any jerk says in Hebrew is called Torah. Some go further and say that anything some idiot with a paper of ordination from three other idiots says is a halacha. [The Oral Law is the two Talmuds, Mechilta, Sifra, Sifri, Tosephta. Five books. Nothing more or less. ]
In any case, the Gra is getting this from a Mishna and a statement in the Jerusalem Talmud.
And that is important because it has become customary to change what the Torah's view is on things in order to make it more compatible with some delusional idiots ideas of what Torah ought to say.
So this idea is money in the bank. We can count on this idea as being accurate, that every word of Torah outweighs all the other mitzvahs.
And when the Talmud says when a mitzvah comes along that can't be done by others one stops learning, the Gra says that means one is allowed to stop learning to do the mitzvah, not that he has to.
And here the Gra is making a lot of sense. For one who is occupied in one mitzvah does not have to do any other mitzvah even if the other mizvah is greater. So it makes sense that one can go and do the other mitzvah if he wants to, but he does not have to.
This is all in the way of introduction. I know the Rambam holds from learning Physics and MetaPhysics as the fulfillment of the command to love and fear God because of how it inspires a person. I just wanted to bring the idea of the Gra as a first axiom and then
And learning Torah is like sacrifices that need to be for the sake of heaven for them to have any value. A sacrifice that is offered with intention to eat it after its time allotted is not just not a mitzvah, but karet [cutting of from ones people].
I am not one to try to decide between these people. But what I would suggest is that if we can't be learning Torah with the kind of התמדה constancy as the Gra was advocating, at least we can put in a couple of hours per day. [For people just starting that would be the written law that is to go through the Old Testament word by word from beginning to end, and the Mishna. upon which the Talmud is based. Also one session with Talmud in depth to begin to get an idea of the depths of the Talmud because that is important at the very beginning of ones learning. If you don't get that right away, you never get it. You find lots of people that think learning Talmud in depth means memorizing lots of commentaries or other nonsense.