While Gyllenhaal has repeatedly brushed off the speculation, he released his first video on the social media platform on Wednesday, with the short clip showing him reading an issue of The Amazing Spider-Man with the caption, "I just realized I'm not playing Spider-Man".
The "Donnie Darko" star just joined Instagram and this was his first post.
The role will be the actor's first major part in a superhero blockbuster film. And, with the current rumour-mill suggesting we'll get a Spider-Man: Far From Home trailer at the weekend, we'll bet good money we'll get to see the character in action (why else do the Jake reveal this week?) when it lands.
In the comic, Mysterio tries to convince Spidey he caused a man's death, The Hollywood Reporter reports, noting that it's unclear if this plot will carry over into the film.
Samsung pushing out Galaxy Note9 Android Pie-based One UI
The report further states that beta builds are now available for Galaxy Note 9 units in the United States, India, and Germany. Once you have registered you will need to download the software manually and it comes in at around 1700MB in size.
Spider-Man creators, Stan Lee and Steve Ditko, introduced Mysterio in the comics in 1964.
The main characters from Homecoming will all return, with Tom Holland of course reprising his role as Peter Parker/Spider-Man, Zendaya returning as Michelle Jones, and Jacob Batalon returning as Ned, while Marissa Tomei will also return as May Parker.
Jon Watts is returning to direct the sequel to Homecoming, which earned $880 million (£656,216,000) worldwide.
Spider-Man: Far From Home was announced a few months ago.