"can I reduce the IRS mileage deduction with the amount of money I receive for gas?"
Yes. I believe Turbotax asks about non-taxable reimbursements.
It is also acceptable to deduct the full mileage amount and report the reimbursements as taxable income. The outcome (net taxable income) will be the same, although one method will show higher pre-tax gross income than the other.
Also, this assumes your client uses an accountable plan and only reimburses you for expenses you prove with receipts. If your client pays you a mileage allowance and does not require proof of mileage driven or gas expenses, this is a non-accountable plan, in which case the client must include the full reimbursement as taxable compensation on your 1099-NEC, and you would deduct the full mileage rate without additional adjustments.
