Yes. If someone has collapsed, is unresponsive, and is gasping or breathing abnormally, treat it as cardiac arrest. Call 999 and start chest compressions straight away. The 999 dispatcher will guide you through what to do. Do not wait to see whether the gasping stops first. Every minute without CPR cuts the chance of survival.
Gasping during cardiac arrest is called agonal breathing, and it is not effective breathing. The person is not getting any oxygen. Bystander hesitation is one of the biggest factors in poor cardiac arrest outcomes in the UK. A common reason for hesitation is uncertainty about whether the person is really not breathing.
If you are unsure, start CPR. The Resuscitation Council UK and every major guideline are clear: if someone is unresponsive and not breathing normally, begin chest compressions. You will not harm a person who turns out to be alive, and you may save the life of someone in cardiac arrest.