Here is how you do it:
Call Delta and ask for a GTS agent (Global Ticketing Support) (or try calling the KL or AF US or Canada phone numbers, as DL agents answer)

Then be prepared with the exact flight details and the fare (look up the fare details and make sure they are selling you the same)

DL/KL/AF can sell any of the group’s fares on their own ticket stock transatlantic…but it will be on DL ticket stock if you use DL gift cards.

Then once they confirm they can sell you the same itinerary at the same price, give them the gift card details for DL