1 ml of GBL is equivalent to 1.6 g (!) of GHB. So weighing 80 kg and aiming for 100 mg / kg, you'd reach that with 5 ml of GBL.
Also, leave the alcohol out there. GBL / GHB is broken down by the same enzymes in the liver that break down alcohol and they occupy themselves with alcohol first. So there's a chance you'll digest the alcohol first and have not enough GBL left in your system as it already passed along.
200 ml of GBL just increases your risk of premature vomiting to a massive extend, I'd strongly advise against. Take the AEs and shoot yourself down with the Benzos, mix in some neuroleptics if you like and take about 20 ml of GBL max. Benzos attack your CNS together with the GBL and respiratory depression will likely kick in soon.
I think the crucial point I see in ODing with substances is balancing the stuff, so that you are sure to evolve the desired physiological reaction and process without triggering your internal biological alarm systems. And when it comes to downing 200 ml of GBL, I'd be afraid that the body recognizes this as a threat, makes you throw it up almost instantly and leaves you with an effective amount of 3 ml or something, actually hightening the risk of passing out without dying. So I'd try to not completely overshoot in terms of lethal doses to sneak the substances in.
Hope it makes sense. I don't know if it's allowed to post links here, but you can find the equivalent doses of BDO / GBL to GHB quite easily.
Good luck. I hope you're successful.
