I think benzos when used in drug cocktails don't really make the cocktail more deadly, rather people include benzos in their cocktails to help them pass out before they can feel the nasty side effects from the other drugs that could make someone panic and try to abort the process. Because of this, taking a massive amount of benzos is unnecessary unless if your tolerance is like stupidly high. You only want to take enough to help you pass out.
Now gabapentin, the chances of CTBing from ODing on Gabapentin is very low, even if you take a massive amount with the help of alcohol and benzos. I can't recommend this method. However, both benzos and gabapentin can be helpful in other drug cocktails (they might also boost the effects of other more deadly drugs), but both of those will not make up the majority bulk of your cocktail.
Really there's only 2 types of drugs that will give you any sort of decent shot of CTBing on ODing. Those are barbiturates and opioids (but only the strong variants). Barbiturates I think have the highest success rate of drug ODing but are also the hardest to get a hold of (Nembutal's suffering a global shortage for example. You can't really get a hold of it online, you have to be lucky to find it in a country that sells it and hope the vendor is willing to sell it to you.). Opioids are easier to get a hold of, but you'll generally need a larger bulk of them and even then, still unfortunately have a lower success rate compared to barbiturates... That is unless if you're lucky enough to get your hands on fentanyl or carfentanil, both of those are stupidly strong (especially carfentanil, which is so strong even particles of it in the air can be deadly to people) and will definitely be fatal to OD on. (It is possible to build up a tolerance to a small amount of fentanyl, but getting your hands on a large amount of it I imagine should still be fatal.)
