giffgaff will send small orders (up to 3 sims) to Spain. The standard order page https://www.giffgaff.com/orders/mgm has Spain in its dropdown, whether you select "send to me" or "send to a friend".
Spain is not currently on the list of restricted destinations at https://www.giffgaff.com/help/articles/whats-giffgaff-doing-about-coronavirus-covid-19. So I presume you can complete the order and sims will be sent there. Though I'd expect them to take maybe a couple of weeks to arrive at the moment.
But make sure anybody that you give the sims to understands that a person activating a sim in Spain they will get only very limited service because they don't have previous service in the UK. EU Roaming is for UK people making "brief visits" to an EU/EEA country. It doesn't apply to those who live permanently outside the UK.
They will of course be useful for anybody planning to visit the UK. But:
- goodybags recur monthly by default. They need to be careful to turn off the recurring option at https://www.giffgaff.com/profile/payment-details if they're only making a short visit
- those who might visit the UK annually need to be careful. Sims will expire after 6 months without making an outgoing call or text, losing any remaining credit. Sending a single 10p text every 6th month will keep them active but it will need to be done regularly.