> Someone on the Beancount thread explained his workflow using tags link transactions to IRS form entries. I would like to try the same on GnuCash, but haven't yet figured out whether tags even exist (haven't had the time to check since)

I don't use tags, but GnuCash does have some built-in reporting features for tax forms.

1. You have to have assigned an account to a tax field (Edit -> Tax Report Options). It's what controls the "tax-related" checkbox when you edit an account.

2. Then you can go to Reports -> Tax Schedule Report & TXF Export to generate a report for the last tax year.

Hopefully this helps!