3 days postpartum drinking 1 gallon of water and i am breastfeeding my baby boy but my breast milk combined with both breasts is only 2-2 1/2 oz and he is still hungry after. is it safe/okay to give him gentle-ease similac ready to feed right away?

You can increase your supply by limiting the duration at each breast to 5 minutes, when baby extracts 80+ % of its content (its pushed out during letdown). Then feed at shorter intervals (like 2 hours while awake. In 2-4 days the kid should build a bigger supply and yor feeds will be more. Feeding formula so soon after birth can trigger an involuntary wean. Get help from a breast feed coach.