Is it permissible to do an action on shabbat which doesn’t violate the 39 categories for a gentile which is paying a monthly salary?

Shalom, One may "work" on Shabbat in a permitted way (such as baby-sitting or waitering) and receive payment for it (after the Shabbat of course) on the condition that the payment is included together with a non-Shabbat work. For example, if you are employed to work as a waiter and are paid for the time on Shabbat together with time spent on Saturday night cleaning up, or for waitering on a weekday, it is permitted. This is true as long as the Shabbat employment is part of the same employment agreement as the weekday work, and you are employed for both of them together as one package. So if you are employed on a monthly basis, with includes non-Shabbat work hours, you may be paid for the whole month including Shabbat hours. Blessings.