Only if you use the SIM more in Spain than you do within the UK.
There is still confusion with the June 2017 EU roaming directive
If you are roaming then virtually all calls between EU member states are included (but please see the notes below).
BEFORE YOU LEAVE THE UK
To set up EU Roaming.
Nice and easy, all that you need to do is enable roaming in your giffgaff profile, if it is not already enabled,www.giffgaff.com/buy/topup
to make international calls / send texts).
2. The EU regulation does not apply to free giffgaff to giffgaff calls where the 'calling' phone needs to be in the UK; the regulation only applies to calls which are normally charged.
3. The EU regulation does not apply to use on-board cruise ships
4. The EU regulation states that it is designed for 'occasional' use and cites a maximum period of 60 days roaming use in any four-month period.
As stated above. you will get a maximum of 60 days inclusive roaming after which you will be on PAYG rates (although these actually reduce to 3.2p per minute 1p per text and 0.078p per MB data if you still have an active goodybag).