Soheil, Baruch she'Kivanta! This is a very important question. A similar question is asked by the Bach in his commentary on the Tur. I will try, bs'd, to explain this, starting from the original sources.

1) The Gemara in Bava Kama 85b tells us that an example of liability for damage of "Sheves" (which is when someone is forced to "sit" at home idly and is unable to go to work and earn money) is where A locked B in a room and prevented B from working. A must pay B what he would have earned if he would not have been locked in the room.

a) On the basis of the above law, the Bach (in his commentary on the Tur, Choshen Mishpat 263:6) asks a question on the Rosh (Rabeinu Asher) here on 20a (beginning of section 6) who cites our Tosfos word for word: "Even if someone drove his friend out of his house and locked the door in front of him [and thereby prevented him from renting out the house], this is considered merely Grama and he is exempt from payment." The question of the Bach is: What is the difference between locking B in the room, where A must pay the salary that B lost, and A locking the house and making B lose the rent money, where the law is that A does not have to recompense the money that B lost?

b) The Bach answers this riddle by making a distinction between whether the damage started immediately or only later on. When A locked B in, the loss commenced immediately because B was prevented from working right away. In contrast, when A locked B outside of his house, B started losing the rent money only afterwards, when he obtains tenants who are now prevented from renting the house.

c) This distinction between damage that starts immediately and damage which only starts later is actually a distinction made by Rabeinu Asher himself, to explain the fine difference between Grama and Garmi. This second passage of Rabeinu Asher is found in the ninth chapter of Bava Kama, near the end of section 13. Rabeinu Asher writes that when the damage starts immediately, this is considered as Garmi and he must pay. Therefore, if B possesses a document which states that C owes him money and A burned this document, this is Garmi and A must pay, because the damage happened immediately since B lost his money on the spot. In contrast, Grama does not start immediately. If A broke a fence in front of B's animal which then went out and did damage, A is exempt because the damage did not start immediately but only later on when B's animal did damage.

d) To summarize, Tosfos (20a, DH Zeh Ein) writes that if A drove B out of his house this is only Grama, because B started losing money only later on when he lost the potential tenants. Garmi is different, because it means that the loss starts right away, such as at the moment that the deed of debt was burned up, at which point B loses his money straight away.

2) Here is a different approach to this question from the Ketzos ha'Choshen on Shulchan Aruch, Choshen Mishpat 333:2:

a) He makes a distinction between a person damaging a person and a person damaging property. The Ketzos ha'Choshen starts with the assumption that if A locks up B's store, thereby preventing B from selling his merchandise, A is exempt from payment because this is Grama (this is, of course, similar to our Tosfos, where A drove B out of his house and locked it up, preventing B from renting out the house). Again, the question is: What is the difference between this case and the case in Bava Kama 85b, where A must pay when he locked B in his house and stopped him from going to work?

b) The Ketzos ha'Choshen explains that there is a difference between locking a person in his house and locking a person out of his house.

We should note first that there are two kinds of "Sheves" payments, two kinds of damages which involve making a person sit at home and lose earning time. The first way is where A injured B. A must pay for the physical damage he did to B and he also must pay for the time that B loses from work. The second way is where A locked B in his room. No physical damage is involved, only lost time from work. This is called "Sheves she'Lo b'Makom Nezek" -- the only loss is "sitting" at home, with no other damage involved.

c) The next stage of the argument is to point out that there is something special about the Halachah of "Sheves": it applies only to damage done to a person. If one damages property there is no such Halachah that one must pay for causing the other person to sit idly. This is why A locking B's store is different from locking B out of his house. When A locked B's store, he damaged his property, but when A locked B in his room he inflicted damage on B himself. It is not physical pain, but he has limited the physical mobility of B, being cooped up in the house all day.

d) The Ketzos ha'Choshen now continues and states that the Halachah is that one is indeed liable for Garmi, even though one is exempt for Grama. However, the Ketzos asserts that the combination of Garmi and Sheves applies only when one damages a person. Therefore, when A locked B in his house he must pay for lost earnings because this is Garmi of Sheves on a person, but when A locked B's store or house, this is damage inflicted on property, and there is no such Halachah of Sheves inflicted on property.

e) Getting back to our Tosfos in Bava Kama 20a: When A drove B out of his house, this is considered as damage on the house, not damage on B, since B cannot now let tenants into his house and accrue the rent. The Halachah is that one is not liable for Garmi when one caused property to sit idly, unavailable for renting.

The current pandemic has made millions worldwide stay at home without work, but we can also look on the bright side since many are taking the opportunity now to learn Torah at home!

Soheil, thank you for sending this very good question.

May we hear good news soon! Pesach Kasher v'Same'ach,

Dovid Bloom