Yes, GitJournal can tag notes, and it stores them as part of the metadata header at the top of markdown notes.

Tagging is one of the paywalled features in the mobile apps. It's such a core feature that paywalling it seems a bit mean, but OTOH it's a great project, and this is the feature that converted me to a paying customer. It was £32 (~$42 USD) - not cheap, but this permanently unlocks current paywalled features, and any added over the next year. Pricing is hard, but I do wonder if the high pricing will ultimately be detrimental.