The most common place for me to find gas leaks is at gas valves. Older style gas valves that aren’t allowed any more today are often referred to as lube valves or plug valves.
These valves are easily identified by a nut or spring on the valve, across from the handle; newer gas valves don’t have these. I would estimate that I find leaks at about one out of every five of these valves. Gate valves, which should only be used for water, are also common offenders.