Gas Detectors

Hi Guys!  Because its been pretty warm out, we left our windows open. But last night we had our standby generator's valve blew. Gas blew into the house and we couldn't smell it. We lost a cat but our dog and other cats are safe. This has gotten us thinking that we need a detector for gas in our house. We woke up at about 6:30 like normal and as we got downstairs and smelled something wrong. We immediately hit the fire button on our control panel and hit the emergency shutoffs for all our gas supplies. As neighbors were getting up, there houses got the smell as well. We have a team here right now cleaning the house from the gas. My question is what gas detector do you guys suggest? We have the NX-8E 192 zone system installed and monitored through a local company. We would like this not to happen again and be alerted if it does. We could not think of what would have happened if it had happened during the day and we were at work or school. Thanks again guys! 

I have a Generac automatic standby generator.  It was installed by a licensed HVAC technician who followed all the required building codes for the installation.  IIRC,  local code requires the generator to be installed at least 10 feet away from any open window.  Otherwise, the window has to be permamently locked. This requirement is minimize the possibility of carbon monoxide exhaust fumes entering the house.  So, was your generator install done according to local building codes?  If not, a gas sensor would NOT be a very good solution.
 

Gas blew into the house and we couldn't smell it.

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Natural gas and propane gas have a distinctive rotten egg smell.  Both gases in their natural state are colorless and odorless.  All gas vendors add a chemical called mercaptan to the gas in order to give it this rotten egg smell.  You will definitely "smell" a gas leak. 
 

We would like this not to happen again and be alerted if it does.

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Prevention is the only real option. If a gas sensor detects gas, you've already got a potential for disaster. One small spark such as turning a light on may cause a major disaster.
 
 
Have you contacted the generator manufacturer?  I'm sure they would be interested in addressing  a generator gas valve issue.
 

There are very common safety gas valves available today that shut off the gas supply when there is excessive flow. These safety valves are so common that its a bit surprising you didn't have one already, but just make sure you get one installed. They are quite inexpensive.