You are fine. Worst case, you severely oxidized the beer (which you didn't), it will take a while for the beer to actually absorb the O2. Right now the O2 is dissolved in the beer, and not affecting any flavors. Over time the beer will absorb the O2 and react to it. The result of the O2 in the beer will be a loss of hop flavor, and a stale taste (cardboard). The solution is to bottle and drink quickly before any of the effects of the O2 appear. With just 1 gallon, I suspect you will not have to worry about any bottles being left for several months.