Your issue is two-fold I believe, part is the smoalking/vaping but the other part is mindset.
Clearly your mindset is not being helped by the choalking. My wife has similar issues with choking and I can get to how she overcomes that.
For some the first breakthrough is wonderful, but I am not so sure that for most it is not equally terrifying. For me it felt like a dentist reaching into my brain, and rummaging around in there whilst I was strapped to the chair. Seeing this as an external entity that means you harm is going to increase the terror. Think of it more as another part of you getting free reign over the parts of you usually fully controlled by your ego. And believe me, an unconscious part of your ego really does not like this at first.
If someone walks into your house, you can either see them as a killer come to get you, or a friend you have invited over. But by taking DMT you already invited them, so don't be so surprised when they show up.
Be assured, nobody is trying to kill you. Some here have been torn limb from limb in those first encounters, those that have often come back the strongest.
So, be ready to invite them/you in, accept them/you and love them/you, or don't do DMT. Although I would say DMT is a great way to learn to do this if you are initially not able to.
Choking is probably the reason for you defensive psychological reaction though - hard to relax and accept when you feel like you are fighting for your life. And I would not expect you to break through whilst running for the toilet, especially if you have choked most of it out. Running to the toilet I would imagine has quite a grounding effect, especially if also psychologically you are trying to block out an assumed killer.
The solution for my wife has been to take many more smaller puffs and not so deep, believe me she still gets there. A desktop vape with a bag helps with this as we can make a more dilute vapour.
If you are used to being a hardened smoker, then perhaps you are burning it, I am not personally familiar with the famed GVG so I cannot help you on method, but if the smell is of burnt plastic then I would guess this may have happened, but really thick spice vapour does take some getting used to for us non-smokers.