Not sure how serious the thread starter is being, but Gas Safety checks have to be carried out by law - if a tenant doesn't give access they are breaking the terms and conditions of their tenancy - the HA can take legal action to foece entry to carry out the test.
It is in the tenant's interest anyway - it's a safety check, to make sure there are no problems with the gas supply.
People who refuse access also raise alarm bells with Housing Management - are they actually living there? are they sub-letting? Which could prompt more investigation.
So, for the sake of you, and any ferrets, or hamsters, or any other creatures that may be livign with you - let them in to do the test.