The only problem with 1 gallon in a 5 g fermenter in the amount of headspace where oxygen can interact with the beer.
Not really a problem during primary ferment as there's enough carbon dioxide produced to create a protective blanket over the beer.
Pick yourself up a couple of growlers. That's what I use when I make test batches.
I split them between growlers with varying dry hopping. Stick a bung and airlock in em and your all set.
Just my way..
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