I've renewed a license for v4, for a community that is 3.4.6, and had gallery. The gallery is not used much so I've only purchased license for the forum.

During the upgrade process, I get errors that the files under webroot/applications/gallery are missing. How do I disable / remove the gallery component so that I can upgrade only the forums?

Thank you.

You may want to reach out to IPS support which will upgrade your community as a courtesy.  But if you want to do it yourself, the easiest would probably be to make sure you uninstall gallery in v3 prior to the upgrade.