The most important characteristic that a drug must have to freely pass from the gastrointestinal tract into the circulation is to be

There are two answers - in salt form and fat soluble, depending on which way you look at it. According to Penn Foster's "How Drugs Work", "No matter what form of the drug is swallowed, the drug must go into the free solution in the stomach or small intestine to pass from the GI tract into circulation. One factor that allows many drugs to go into free solution in the GI tract is that the drug is in a salt form within the commercial preparation. Salts of drugs dissolve readily in stomach liquid."

Also quoted from "How Drugs Work", "The portal blood supply is joined to the small intestine by capillaries connecting to the gastrointestinal mucosal cells. In some cases, it isn’t easy to get through these mucosal cells because there’s a selective process that attempts to allow only important substances through. In the case of drugs, it has been found that the fat-soluble molecules (lipid-soluble drugs) are absorbed through these mucosal cells very well."