Unrecognized gains/losses are amortized over active employees future service. When a plan becomes frozen are gain/losses fully recognized? Or do you continue to amortize based on future service to retirement even though no additional service is recognized for benefit accrual?

When a Plan becomes frozen, it is generally considered a Curtailment and using the FASB worksheets you will normally recognize much if not all of the existing gains/losses and transition obligations/assets as well.
Each year thereafter, you still can generate gains and losses requiring amortization.

I believe you would still amortise gains and losses over the future expected working lifetime of employees expected to receive benefits from the plan. If all or most of the employees become inactive, I believe you switch to expected life expectancy for amortisation.