Gabriel's call comes amid indications by Turkey's prime minister that there may soon be movement in the high-profile case that has become a major irritant in the two countries' relations.
Deniz Yucel, a 44-year-old correspondent for the daily Die Welt newspaper, still has not been formally charged since his arrest last year in Istanbul on terror allegations and espionage. Yucel, who has both German and Turkish citizenship, is one of six German citizens imprisoned in Turkey for what Germany considers political reasons.
Turkish President Recep Tayyip Erdogan has in the past alleged that the reporter was a German spy and a "representative" of the Kurdish PKK rebel group - all of which the newspaper denies.
Vigils were held Wednesday in Yucel's hometown of Floersheim, in central Germany, and many papers marked the one-year anniversary of Yucel's imprisonment with big pictures and stories about him on their front pages.
In Berlin, supporters drove dozens of cars decorated with "Free Deniz" posters in a long procession through the capital, honking and slowing down traffic to call for Yucel's release.
Gabriel said Wednesday during a trip to Serbia that in several recent conversations with his Turkish counterparts he asked that "the Turkish judiciary speeds up the trial."
He also said in a statement late Tuesday that Germany thinks that a fair trial can only end in the release of Yucel - a stance that the German government has made clear many times in the past year.
Turkish Prime Minister Binali Yildirim, who will meet Merkel in Berlin on Thursday, said on German ARD Television late Tuesday: "I hope that he will be released soon. I think there will be a development shortly."
"At least he will appear in court and every trial is a chance for him to be released," Yildirim added, according to the German news agency dpa.
Yucel's case is certain to be part of talks Merkel and Yildirim will hold at the Chancellery Thursday.
Copyright 2018 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.