Since codes for new gift cards are automatically created in a read-only field, the only way to import codes is to 

1) remove the read-only functionality with a customization.

from odoo import models, fields, api

class GiftCard(models.Model):
    _inherit = "gift.card"

    code = fields.Char(default=lambda x: x._generate_code(), required=True, readonly=False, copy=False)

2) import