A Christian pastor from North Carolina living in Turkey for 23 years, Brunson has been indicted on charges of helping the group that Turkey holds responsible for the failed 2016 coup against President Tayyip Erdogan.
Turkey has also been at loggerheads with Washington over U.S. support for the Syrian Kurdish YPG militia, which Ankara considers a terrorist organisation linked to Kurdish militants PKK, waging a decades-long insurgency on Turkish soil.
The American Center for Law and Justice, a conservative Christian group in the U.S., has called Brunson a "hostage of the Turkish government".
But Erdogan fired back at Washington, demanding that the USA first return Gulen. That has created festering frustration in the Turkish government, which has hunted down tens of thousands of alleged Gulen supporters and either imprisoned them or fired them from government jobs.
"Give him (Gulen) to us, and we will try (Brunson) and return him", Erdogan said previous year.
Turkey has submitted an extradition request to the USA for Gulen, who lives in Pennsylvania, but so far it not been granted.More news: #NotJustanUncleAnymore: John Stamos is a dad
Brunson has been the pastor of Izmir Resurrection Church, serving a small Protestant congregation in Turkey's third largest city.
Washington has stepped up efforts recently to secure the release of Brunson, who faces 35 years in prison and is also accused of trying to convert Turks to Christianity.
A copy of Brunson's indictment seen by Reuters accuses him of working both with Gulen's network and the Kurdistan Workers Party (PKK) which has fought an insurgency in southeast Turkey and is designated a terrorist group by the United States and European Union.
The indictment says the charges are based on evidence obtained from Brunson's phone, as well as information from witnesses.
"We think we can debunk these claims tomorrow", Brunson's lawyer said.
Turkey supported the air strikes by US, British and French forces, saying the move sent a message to Mr Assad.