The gas/electric decision would be based on what you PRIMARILY do with it. It isn't strictly a power issue, though it can be. If you want it to wash cars, you don't want a gas. You'll have to turn the motor on and off too much because the unit is not designed to run for more than a minute or so without the pump engaged. Basically, a gas unit is designed to work while running UNLESS you're changing hoses or attachments.
With that said, if you're looking for a 5 gpm electric model, to the extent they exist, they're going to be VERY expensive. Also, if you must have mobility and either can't or don't want to mess with cords, then the gas wins out. However, the gas requires much more maintenance and, everything else being equal, probably won't last as long. My cheapo electric just keeps going and going. My gas works fine too but I've had to mess with the carb a few times.
At the consumer end of electrics, I recommend AR Blues. Most of the brand names you've heard of -- Sun Joe, Greenworks, heck, even Ryobi -- are good machines. I think the ARB is the most consistent. These are all going to be about 1.5 GPM units or less -- most 1.2 or less. If you want a more powerful electric, check out the DeWalt 1500. It is a 2 GPM. It is more, like about 550-600 at Home Depot (and elsewhere, I'm sure) but it already has a good hose and is quick-connect ready. The consumer electrics all come with M22 fittings that I recommend you change to quick connect at some point. You'll need to buy a new hose and gun but it will greatly improve your experience. I've written extensively about this on other threads.
If you feel a gas is needed, I strongly recommend you get at least a 3.5 GPM model -- a 3.2 at the very least. 4.0 is considered minimum pro level. The 2.x models are OK and while they're much better pump wise than they used to be, if you decide to do some higher end work like renting a pro surface cleaner, then the bigger machine will do it much better.