When you run a site collection health check before upgrading to SharePoint 2013, SharePoint will attempt to rebuild any missing galleries, such as a web part gallery, automatically. No action is necessary on your part if this succeeds.
If SharePoint is unable to rebuild a missing gallery, it may mean that the gallery has become corrupted in some way. In this case, the missing galleries will be listed on the health check results page.
To fix this issue, it may be necessary to contact your Farm Administrator or support and have them delete the gallery and recreate it before proceeding with the upgrade. If you have any custom data stored in the gallery, you will need to back up the data before deleting it.