Hey man, just make sure that we keep the discussion focused solely on Harm Reduction and not recreational usage. I'm not accusing you, just saying. The thing with Gabapentinoids, is that they do not seem to have an impact on respiratory depression, but I cannot state with absolute certainty that this is the case with Phenibut or any Gabapentinoids for that matter. There is not a lot of information available concerning the combined usage of Gabapentinoids and Opioids.
I have extensive experience with the combination though. Most of my experience is with Gabapentin (Neurontin) and/or Pregabalin (Lyrica), however, not to mention the fact that Phenibut is not even considered a medication in the United States, it's considered a nutritional supplement, thus, there is just not a lot of study dedicated to its usage as such. What I can tell you, is that their is a definite synergy between the two. You will be more intoxicated than if you had taken them separately.
As I've stated, I don't know for sure about the respiratory depression, but I know you can get severely fucked up by the combination; slurred words, unsteady gait, subjective feeling of lost clarity of thought etc. In civilized society, there are typically consequences of some kind for being that intoxicated, so whether respiratory depression is a factor or not, you don't want to be getting that intoxicated anyway.
There are a lot of hardcore Phenibut users here in the community, so just give it a minute and I'm sure you'll experience a flood of opinions. Please just remember what I said and don't let this turn into a discussion of recreational usage or how to get the most fucked up, lest I close it.