You’re welcome.

You are correct about your impression of the term “insert”. That can be anything from a preamp, distortion, delay pedal etc.

What I refer to as 4CM can also be thought of as Pre/Post, where an amplifier has an FX loop. The “Pre” path goes directly to the input of the amp, the “Post” path is the amp’s FX loop. This is shown here: https://cdn.eventideaudio.com/manuals/h90/1.5.2/content/setup/pre-post.html

Dual Mode with 4CM or Pre/Post may be desirable with your setup if you wanted the H90’s effects on a separate path from the GT1000. You could have the GT1000 + H90 effects on Path 1 with nothing on path 2, or GT1000 on Path 1 and H90 on Path 2. You would need some way to mix the signals together.

You could also process 2 instruments at once, for example, guitar on Path 1 and synth on path 2.

You should use whatever works for your setup. The GT1000 on the insert path of the H90 sounds like a good option.