Is it safe to leave a gas oven on unattended?
The US Fire Administration clearly recommends not leaving cooking appliances unattended when no one is home:

Based on 2006-2010 annual averages:

Unattended cooking was by far the leading contributing factor in home cooking fires.
[...]
Ranges accounted for the largest share (58%) of home cooking fire incidents. Ovens accounted for 16%.


At the time this related answer was written, the Fire Administration said (they have since updated the page):

The leading cause of fires in the kitchen is unattended cooking.

Stay in the kitchen when you are frying, grilling, or broiling food. If you leave the kitchen for even a short period of time, turn off the
stove.
If you are simmering, baking, roasting, or boiling food, check it regularly, remain in the home while food is cooking, and use a timer
to remind you that you're cooking.
Stay alert! To prevent cooking fires, you have to be alert. You won't be if you are sleepy, have been drinking alcohol, or have taken
medicine that makes you drowsy.