Your probably best going with a custom built PC for better performance. Choose your own parts because prebuilt systems (gaming or not) are pretty bad combos but it's your choice really try PC part picker or research about getting good parts
Not only that you can exact your budget yours is pretty high but it's really your choice if your going to buy a pre built gaming pc
For that kind of money, assuming you're talking US online prices for the system only and not a monitor, I would try to look for something with a graphics card that offers at least GTX 1060 or RX 480 / 570 / 580 -level performance, and preferably a 6-core Ryzen or Intel CPU (though a recent quad-core might be okay). For gaming at 1080p resolution, you probably wouldn't want to cheap out any more on the graphics card. Systems with a decent graphics card and processor in this price range will most likely only have 8GB of RAM though, which isn't ideal for a few of the newest games, though nearly all games will still run rather well on 8GB for the time being. You might be in need of a RAM upgrade within the next couple years though.
Something like this (which comes up as "Amazon's Choice" when searching for "Gaming PC" there) would probably be decent enough, and you probably wouldn't currently be able to get much better performance out of a pre-built system for $800. It might be worth looking around in case any better options pop up though...
Some of those "Free Upgrades" in that iBuyPower system are likely needlessly adding cost to the system, even if they claim that they are "free". Why would a "base system" need a 120mm Corsair H60 water cooler? They could probably cut more than $50 off the cost of the system just by omitting that and sticking with the stock cooler. Likewise, I would prioritize a move up to a better graphics card before a "free" upgrade from a Ryzen 2600 to a 2600X, especially if they're replacing the stock cooler, the 2600 could be overclocked pretty much just as high on the included X470 motherboard. And again, even a B450 would allow for overclocking on a budget. And yet there's still only 8GB of DDR4-2400 RAM and a 1050 Ti despite the system being over $1000. The base configuration seems a bit unbalanced, and balancing it involves increasing the price further. At least in that CyberPower system at Amazon, they prioritized getting the most gaming performance for the money. The iBuyPower systems are probably decent enough, but they clearly seem to be targeting higher price points.