Is it obligatory to disburse usurious gains to the party specified by the one who gave them, or is it permissible to spend them on other Muslim interests?

These benefits are to be spent on the general welfare and interests of Muslims. Their holder does not own them but is required to spend them in the aforementioned manner. So, if you spend them in their proper place, you have done well. You are not obliged to spend them in the specific direction that was designated for you, especially if the direction you spent them in is of more general benefit and greater advantage to Muslims.