"the gift was deposited into my Fidelity account, and I received a 1099-B for it"
You need to provide more detail of what's going on here.
That sounds like the gift was not cash, but securities of some kind. And the fidelity account was something other than a 529 plan. I don't think you can contribute securities directly to a 529 plan.
If you cashed stocks or other securities and then contributed the money to a 529 plan, then yes, you have a reportable capital gain. But, that is a totally separate transaction from the contribution to the 529 plan.
It's a common tax maneuver to gift a relative, in a lower tax bracket, appreciated securities and let him sell them and report the gain.
