I have tested the gift card feature of Odoo v15.
scenario as follows:
Company currency in EUR
POS currency in USD
I sell 100 USD of gift card.
Gift card that is generated is in company currency EUR 87.39
When I use the gift card to pay something. It gives me only 87.39 USD
Only the amount sold is being recorded. Odoo does not remember at which currency the gift card was sold.