It sounds like they did a year end adjustment to your withholding to make up for under withholding earlier in the year. What explanation is Gusto giving you?
Q. How can I calculate if this is correct?
A. From the tax tables.
Q. Can they use my paycheck to pay taxes beyond what is already taken out?
A. It's unusual, but maybe, depending on what taxes are involved (federal, state, local, FICA) and why there was under withholding earlier. You may have to wail until you file your returns to get any over withholding refunded. If excess FICA (social security and medicare) were withheld, you cannot get that refunded by filing a tax return. You have to get that from your employer/Gusto.
