Changing the volume mid-track GTP4

Once you have created the envelope, you need to add nodes, which are essentially pivot points for envelope changes.
Right click on the line created with the envelope.
Select Add Node.
In your situation, I would add two(a few beats apart) where the passage starts, and two  where it ends.
Drag the section between the pairs up. You should have an increase in volume only in that section.
Without the nodes, you will drag the entire envelope up or down, as you have seen.

Glad to help.
I would suggest reading the Help files. There's stuff in there that you may not even thing to ask about, but is very useful.
And push buttons, see what happens. Plan on trashing a project or two.
If you have something you like, Save As. Then experiment some more.
Once you get the hang of it, it's pretty easy to work with, but the first few weeks can be a bit rough.