I did something similar, using ACF on the CPT with a little JS to let the card configurator put text boxes as overlays on the image, so that the form submitter could write a message that would get placed in along with their name / relation.
All that could be done without Gravity Flow. But some neat enhancements that you could add with it in the mix:
Approval Step or User input step to let an admin / senders manager / etc chose to add their own additional comment to the card or include a gift card / other recognition component.
Use an outgoing webhook step to store the generated card to an external service.
Setup scheduled reminder notifications in a “Remember when…” concept.