Use a Quaternion as a float?

I must make a few assumptions to answer. First, your post doesn't include the quaternion from the gyro, but I think it a safe assumption that the eulerAngles property of that quaternion provides appropriate information.

The 3rd parameter of transform.RotateAround takes an angle in degrees. There are two situations I can infer from your description, and only you can choose which works. First, you may need to instantaneously align the transform to the value given by the gyro. The second is that you may want to gradually move toward the value given by the gyro (animating the move). Only in the second approach do you use the "Time.deltaTime" value to perform that animation, but not likely the way you've given in your post.

Further, I must assume that the axis created in your example is the axis of interest, and is also the axis from which you take data from the gyro (the Y axis, as it appears). If that assumption is correct, then you want the "y" rotation from the gyro's eulerAngles property as the angle you require. Note, however, that the orientation of this value may not be exactly as you expect. It could be the direction of a positive change in the angle is counterclockwise, while you might expect it to be clockwise (as if the gyro is mounted upside down). You must correct for that. Next, gyro's typically can't provide absolute positions. They're not compasses. They provide, instead, a relative rotation in a particular sample. I have no idea what your incoming data actually represents, but depending on how "raw" the data coming from the software interface to your gyro, that value may require a "perspective adjustment", like, perhaps, inverting the direction of the rotation, or "where is noon" (or where is zero - alignment).

Given that, the third parameter comes from something like gyro.Rotation.eulerAngles.y. That's the basics of this. The rest is a matter of refinement for your particular intention, which I can't divine from this information.