Tthe Gentlemen's Agreement was between the United States and Japan. What concession did each side make?

It was:

Japan would greatly reduce the number of passports it issued for emigrants to America.

The Gentlemen's Agreement was a response to a growing hostility between the US and Japanese immigrants. In order to stabilize the relationship with Japan, President Roosevelt promised that the Japanese children and people would be subject to segregation and that the order made by the San Francisco school board to segregate the children would be withdrawn. Japan on the other hand promised to deny passports to workers who wanted to enter the United States and the US had to right not allow the Japanese workers who had passports for other countries.