Vive Tracker Rotation issue or is it really gimble lock ?

For a research I am getting data from multiple vive trackers 2.0. Well, in my case let’s take one tracker as an example.

When tracker rotates around some axis let’s say y axis. the other axes jump after some angle of rotation y. Even if I don’t rotate other axes. I have been told that it is because of gimble lock and I agree on it.

This question has been asked by some other people on forum and I found exact problem that a user was facing as I do. Someone has answered his question and it looked like a good solution for me but there is one thing I didn’t understand.

And I thought some more experienced people could lead me on that,

The question and answer is in the attachment.

I understood the answer given until the point where he says, Multiply the Quat’s together and convert back to rot. Converting to rot is alright. But what He means by Multiplying the Quat’s together? I mean like a cross product of all axes together for each x,y,z and than putting them back as rotation ?

Can someone give me direct answer for that ? Thank you. You may also find the guy’s blueprint in the attachment.

Thanks again.