Metin Topuz, a Turkish citizen and liaison for the US Drug Enforcement Administration (DEA), was arrested in 2017.
Topuz has denied all allegations against him and told the court on Monday that all his contacts were "part of my job", an AFP reporter said.
"I am not a decision maker. I have done what I was ordered to do" by my boss at the DEA, he said.
His defence lawyer presented a list of his phone calls with police, saying they were entirely work-related.
US embassy and consular officials, who have dismissed the charges as being without merit, were also at the courthouse to observe proceedings.
The trial comes at a time when the NATO allies are increasingly at odds over Syria, Turkey's decision to buy a Russian missile defence system and the US refusal to extradite Gulen.
According to his indictment, Topuz is accused of espionage as well as arranging arms trafficking through exchanges on the WhatsApp messaging service.
Since the failed 2016 coup against Erdogan, tens of thousands of people have been arrested over suspected ties to Gulen and more than 100,000 people have been sacked or suspended from public sector jobs. Gulen rejects the coup accusations.
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