Who is Robert Gilman, former US Marine released from ‘wrongful detention’ in a Russian prison after more than 4 years?

From the publisher

Robert Gilman, a US military veteran imprisoned in Russia since 2022, was released on humanitarian grounds after his health deteriorated. US President Trump confirmed his return home, stating no exchange occurred and that Gilman requested a cheeseburger upon arrival.

The source's own excerpt, unedited