You can optionally specify an expiry date for a gallery, which means the gallery will no longer be accessible to your clients once it expires. To change the expiry date of a gallery, select the "Expires On" dropdown in the gallery's settings modal.
You can also specify a default expiry date for galleries in one of two ways:
In a gallery template, you can choose the default expiration that'll be used when you apply that template (i.e., 7 days, 30 days, 90 days). This will be relative to the date you make the gallery when you're using that template.
You can specify a global gallery default expiry date (when not using a template) in Settings > Account > Account Defaults.