Is this the model you have (Pre-Tell 2 model 7719)? If so, it is supposed to Flash when On and gas valve is open according to the manual.
This model and company is out of business, so if you need to replace it, I found this cross-over list that may help. Also, it was said that the gas valve (at the tank) is no longer required, so if you have to replace the detector you could eliminate the shut-off valve at tank (I'm not sure if I would want to, but safety-wise it is not required).