I doubt a 600VA UPS can handle a gaming PC well, unless it is like i3 + 1650 or something. You mentioned 300W load - did you measure it via a wattmeter or something at plug point? Did you include a monitor?
Don't check HWinfo or similar software for numbers. My GPU (~200W) + CPU (~60W) totals to 260W while gaming but my PC's actual power draw is 350-380W on load + additional 25W for monitor, measured at the plug point where UPS connects. My PC's power draw for non-gaming tasks like web browsing is 180-200W (incl. 25W of monitor).
