Gonti, Night Minister is worded in two paragraphs, first being:

Whenever a player casts a spell they don't own, that player creates a Treasure token.

Whenever a creature deals combat damage to one of your opponents, its controller looks at the top card of that opponent's library and exiles it face down. They may play that card for as long as it remains exiled. Mana of any type can be spent to cast a spell this way.

However, in MTG Arena the card ONLY triggers treasure tokens if you cast cards exiled with Gonti, Night Minister from opponent. If you cast a card you have received through another means, e.g. Rev, Tithe Collector - you will NOT create another treasure token via Gonti, Night Minister - even though you should.