Scope
I have one or two existing regular plans, can I send them as gift subscriptions? 

I don't want to have separate gift plans, I want people to gift the regular plans.

Solution
Currently, it is not supported. A gift plan can only be bought as a gift plan. A normal plan cannot be used as a gift plan. A gift plan checkout will be different, and it will ask for a recipient's email. 

Screenshot for reference: