Gelael, 20, will be on duty in Singapore, Malaysia, the USA and Mexico - none of which clash with his current efforts competing in F2. The Indonesian already has previous experience with Toro Rosso, having tested for them earlier this year in both Bahrain and Hungary.
Team principal Franz Tost said those tests had helped convince of Gelael's abilities, "During the tests in Bahrain and Budapest Sean performed very well, helping the team in a very professional way, with a mature approach to the complex task of providing engineers with useful feedback.
"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."
Gelael added: "What an amazing opportunity! First of all, I want to thank everyone who made this possible and most importantly for the belief they have shown in me.
"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 said they will decide which driver Gelael replaces in each session 'in due course'.