“On the house” is an expression. The house means the owners of an establishment. So if a bartender gives you a drink, “on the house”, it means that you are getting the drink for free.
In this case you were given a Thank you for free, which is ironic since they don’t cost anything to give.
Yes I know that’s why I mentioned (most of you might know but when some one new joins they don’t know about thank you) like I didn’t knew that there is a thank you button at first.
I think it’s a matter of interpretation. I think it does cost you something, but the cost is not literally monetary. For those who believe that time is money (as I do), even if it takes two seconds to hit the “thank you” button, he may have just spent two seconds thanking a post that he would not usually give a “thank you.”
In Gambit’s case, he already knew about how and why to give a “thank you,” so he didn’t gain anything (new information, or a laugh for example) in exchange for that “thank you.” Therefore, the “thank you” is considered “free.” He was being nice and giving one just because.
Sometimes, if you just don’t get something, it might not be confusing for everyone. But, I realize you had some trouble with it. I read it again, and it makes perfect sense to me. It happens.