If you have a 3 gallon keg, you're golden. If you have a 5, that's a lot of head space you'll have to purge with CO2.
You can put all 5 gallons into the keg after priming, purge the headspace with CO2, and let it carb up naturally. Just make sure you use a carbing calculator - priming in keg uses less sugar - and make sure to refrigerate before serving. Keg conditioned beer needs to be chilled well in order for all that CO2 to dissolve.
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