Is it really necessary to create a separate gallery for each subpart that has 2 to 4 pages instead of having one gallery for the whole thing?

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I have no idea how many parts there'll be

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Oh, I didn't expect you to, I just meant you could have updated the original 2.1 gallery with the new subparts. Though judging by your response you probably don't know how.
For future reference, you can update your galleries by going on Uploads -> [Name of your gallery] -> Create new version.

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oh shoot, thanks for the advice, I'll create a new version of the 2.1 gallery with everything once a new part comes out

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