In 2 days, must i tell the dental specialist i will see for a 2nd opinion that i'm currently on an antibiotic for gum infection by the other dentist?

Specialist: I assume the specialist is a Periodontist. Good for you. Between your Dentist and your new Periodontist they will get your infection under control. Please follow their instructions to the letter.

Gum infection: Yes. Always provide a complete medical/dental history, including any medications you are currently taking.

You must tell: You should always update each doctor about current medications including most over-the counter.

Yes: Medications will alter true diagnostic testing in absence of something obvious.