Gassing it shouldn't make a difference, as it's CO2 you're adding. The primary problem associated with kegs once they're "opened" is when people are using a party pump on their keg. This pumps in air instead of CO2, which introduces oxygen, which causes oxidation which will spoil the beer in a day or two. A normal kegging system doesn't have this problem, and thus the beer should last as long as it would normally if you had bottled it.