Is it permissible for the wife to save a portion of the household allowance to pay off her gambling husband's debt, and is it permissible for the husband to count the value of a gift he gave to his wife as part of the money he borrowed from her?

The basic principle is that it is not permissible for a wife to take from her husband's money without his knowledge, except in two cases: taking the obligatory maintenance if he is stingy, or taking the amount of a right or debt that he refuses to pay, provided that the taking does not exceed the right, and that it does not lead to scandal or punishment, and that it is impossible to obtain the right through judicial means.

As for the husband retracting a gift after it has been received, it is forbidden by Sharia, unless the gift was for a specific purpose (such as good companionship) and that purpose was not achieved, or if the gift was intended as compensation and it was not received, in which case he may retract it. And one who retracts his gift is like a dog that returns to its vomit.