FWIW, an inverter generator can put out a good pure sine wave with minimal distortion, it's just that inverters that do so are more expensive. It seems to be the case that most inverter generators actually do have high-quality inverters.You should be able to charge a Bolt EV using a gasoline generator provided you use the correct type of generator. I would presume you would want to do a level 2 charge system; level 1 just takes far too long and level 3 is very expensive—the charge system and the required 50kW generator would cost almost as much as a Bolt and be almost as large and heavy.
A level 2 charge system provides 7.5kW or put another way, 32A at 240V. This is equivalent to a household clothes dryer outlet or what is often called an “RV Power Outlet”. With this power, you gain about 32 miles per hour of charge so to completely charge an almost empty Bolt battery requires about 7 hours. So this defines the size of gasoline generator you need to use. I would use a generator rated at 8000 Watts continuous. That’s a large generator, perhaps $1000, 250 pounds and the size of a small mini-dorm refrigerator. If you use something smaller, like a 3000 Watt generator, your charge time will go longer proportionally. Also, you need to look at fuel usage. Many generators will specify they can run for 6 or 8 hours on a tank when delivering 50% load. So if you use an 8000 Watt generator at full load (to fully charge the battery in 7 hours) you will have to refill the fuel tank once or twice during the charge cycle. Also, be sure the generator is rated to run continuously at full load for that long a time and won’t overheat.
Generators put out a “sine wave” which is the same kind of waveform you get from an electric utility (which also is produced by a generator—a very large one). Conversely, an “inverter” delivers a square wave or perhaps a “stepped square wave”. This may also be referred to as a “dirty” signal and can cause problems with many loads. The charging system inside the Bolt expects a “pure” sine wave and may have problems with power that is not a sinewave, such as an inverter delivers. I would certainly not use a generator that did not deliver a clean sinewave. How clean or pure the waveform is, that is how close to a perfect sinewave is specified by how much “Total Harmonic Distortion” is present. 0% is perfect and you won’t find nor do you need that clean. 5% THD is clean enough and good generators will have a number like that.
So to summarize, you need a 8000 Watt gasoline or propane generator (not an inverter-type), a level 2 charge station, and the proper AC connector or adapter to connect the two. The most common type of AC connector is an NEMA 14-50 but there are adapters to the other NEMA connectors used for a 240V 30, 40, or 50 Amp circuit.
The issue with non-inverter generators is that they're much louder.
The Honda EU7000is and Yamaha EF6300iSDE generators are big fairly quiet 240V inverter generators. They can deliver 5500 W continuous, which is 23 amps. That's easily enough for an overnight charge, but huge and possibly overkill, since it costs almost $4000.
It would be be cool for someone to work out the cheapest route for getting meaningful power into a Bolt for the least money but with least risk of whatever the manual is warning against when it cautions not to use a generator.