You guys are way overthinking this. Its not exactly rocket science.
I've been in many hundreds of basins over the past 30 years as an inspector. Theres no poison gas, no trolls, no boogie men in there. Unless its really deep there should be no problem getting the light back.
The manhole cover near each basin should be a cleanout. Its required in most places when there is a junction of some sort. In this case there seems to be a line running down the center of the street with an outlet in each basin draining into that pipe.
Poison gas? Not likely unless either something odd is going on or you actually climb down into a sewer pipe, which this is not, or a deep basin sump.
Unless your system is decades old sewer lines are not connected to storm drains. They are generally required to be separate to prevent sewage overflow in the event of a heavy rain that overwhelms the drain system. The most you are likely to encounter is a slight whiff of methane from decomposing vegetable matter in the basin sump. Remember, this is not a pipe, its a shallow open top basin with plenty of air circulation, not a place you would normally find a lot of poison gas.
Check the depth of the sump, chances are its not deep.
Once you determine how deep the basin is, pull off the grate and take a closer look. If you have another bright light, you should be able to see the flashlight in the water if the basin isn't too deep.
Once you find it, reach in and grab the flashlight using whatever means you have available. Its only water, albeit somewhat dirty water. If you are a germaphobe get a pair of long latex gloves of the sort used by some to field dress deer, the kind that cover your whole arm. Use a pair of tongs to grab the light. The most dangerous thing you are likely to encounter in there is a hypodermic needle. Don't go fishing around with your bare hands.
Bottom line, if you want the light back that bad you have to actually do something to retreive it.