All depends on what you mean by "potentate" for the point of this will just go with it making you nod harder, in which case:
Gabapentin+Opiates: Yes. Gabapentin works on gaba receptors, as do benzo's. Some will argue over how good gabapentin is (it is NOT near benzo's, and is uncontrolled) but it does help with anxiety/sedation similar to benzo's, and can increase the nod.
NyQuil + H: Yes. Depends on what you mean by NyQuil, I'm assuming you mean the antihistamine (Doxy Succinate). Generally all antihistamines + heroin will be a potentiator and increase the nod. Be careful that you don't do too much because passing out when really jammed is not good. Doxy/Hydroxyzine/Diphenhydramine will all potentate H. Also, be careful with NyQuil, as generally the antihistamines are safe alone, NyQuil has other shit that you don't want (Depending on brand: Tylenol, pseduoE., guaffen, etc)
OC and Tramadol: Yes. Tramadol can essentially be considered an opiate. At the most basic level if you have more room for more opiates, then more opiates will make you more jammed. (hope this made sense, if u don't get it ask and Ill explain).