The Indonesian driver, who is 20th in the Formula 2 standings, drove for Toro Rosso at the post-Bahrain and Hungarian GP tests earlier this year.
Gelael’s first outing will come in Singapore with further runs in Malaysia, the United States and Mexio.
“First of all, I want to thank everyone who made this possible and most importantly for the belief they have shown in me,” he said.
“It is an honour to be driving in Formula 1, starting in Singapore and Malaysia, two venues that are so close to my home country, Indonesia.
“I will use the opportunity to absorb like a sponge everything I can and learn as much as possible.
“I hope I can do a good job for Scuderia Toro Rosso and provide the team with good data from these important first sessions.”
Toro Rosso team boss Franz Tost added: “During the tests in Bahrain and Budapest Sean performed very well, helping the team in a very professional way, with a mature approach.
“Everyone within the team was impressed by his performance, his technical understanding and his commitment.
“Therefore, we are happy that he will be driving the FP1 sessions for us at these four grands prix; I am sure he will do an excellent and useful job for the team.”