What happens when a multi-fuel generator reaches capacity on LPG or NG?
I have a multi-fuel generator rated for 7.5KW on gasoline, 5.5KW on natural gas.

That means that

the alternator is rated at ≥7.5 kW.
the engine can supply enough mechanical energy to generate 7.5 kW when running on gasoline.
the engine can supply enough mechanical energy to generate 5.5 kW when running on natural gas.


It has a 30A circuit breaker that will trip if demand exceeds capacity on gasoline

The circuit breaker has no idea what is generating mechanical power. It will trip when the current exceeds 30 A for a certain time.

I would like to understand in some detail what happens if demand exceeds capacity on natural gas. The manufacturer says the breaker will trip. Is that right? How will a 30A breaker trip at what should be around 22A at max power on NG?

See above.

Common advice on some other forums is that the generator will overheat. Will it? Neither the engine nor alternator are running at the actual full power capacity designed for gasoline.

The engine may overheat. The generator should run normally if you don't exceed the rated electrical demand for the fuel being used.

Will voltage drop?

If you overload the motor then frequency will drop due to mechanical loading. It is likely that the voltage will drop too.

Presumably this could harm anything powered by it ... not just sensitive electronics but anything.

Some things - lights, for example - will just go dim and won't be harmed. Others such as motors may stall and burn out. Electronics will vary.