This is a walkthrough for Gabriel Knight: Sins of the Fathers - 20th anniversary.
Not everything I describe will be necessary to do. It won't harm to do them, though, and hopefully, you're playing the game for the story and the atmosphere as well as any other reason.
You use items by selecting them in your inventory, clicking on whatever you want to use the item on, and selecting the object from your inventory that you just selected, that should appear as one of the icons.
By clicking the medallion in the upper right, by the investigation points, you can see interactable spots in the area you're in.
In general, look at everything, and try to manipulate everything.
When talking to people, the chat option usually goes grey when there is nothing more to ask about. Not always, though, but those cases will be obvious, as you'll keep asking the same question over and over. Sometimes, I'll write that you should ask about a subject that might not initially show up as an alternative. Keep asking about other things, and the subject should become available.
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