Sorry but in a word....No!
Gib. MOTS are only valid for Gib reg vehicles in exactly the same way as Spanish ITV certs are only legally valid for Spanish reg vehicles.
The only valid MOT cert for a UK reg vehicle is one issued in the UK. You should also be made aware that a SORN cert has no validity outside the UK either. Once the UK road tax runs out you are really stymied because DVLA's enforcement branch called the Continuous Registration Branch will be after you too with fines for non-payment of Road Tax.
If your UK MOT has expired then you are about to join the ranks of the hundreds or thousands of UK reg vehicles that are illegal in Spain and which the Guardia Civil Trafficos are currently having a purge on, particularly in the holiday resorts and Brit enclaves in Spain. They can (and regularly do,) confiscate such vehicles and have them scrapped and fine the owners for non-compliance with Spanish Law.
The only option for an untaxed, un-MOT'd vehicle in Spain is to have it transported to the UK, directly to an MOT Station with a pre-made appointment.
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