http://www.nolo.com/legal-encyclopedia/free-books/renters-rights-book/chapter7-2.html
Without gas, your unit is not habitable for 5 days. The tenant has the right to withhold 5 days of rent (and you cannot evict them), or pay full rent but sue you for 5 days of rent. I think it is best for you to not collect rent for those 5 days.
Collect your evidence (your communication with gas company, and the lease, that month's reduced rent), you can either ask or sue the gas company for damage for their mistake. You might try ask the company or its regulating government body first on how to pay out your damage.
