I would caution against dropping in GPP work on the off days--very quickly you'll find you have no more off days and your recovery and therefore progress will suffer greatly.
If you feel you MUST continue doing GPP work, I would instead experiment with doing it on the same days as the SS training--after it. Start easy to make sure you're not overdoing it and then add more if you can. But keep in mind that by and large, an individual can only make remarkable progress in one area at a time; that is, if your goal is gaining significant strength, you'd be well advised to really focus on that goal and not worry too much about a loss of conditioning. In my experience, not only will your newly developed strength actually contribute to your metabolic capacity, but that conditioning will return relatively quickly to high levels. Using myself as an example, I've done zero metabolic conditioning for around a year (o-lifting only). I got suckered into participating in the CF prostate cancer fight gone bad deal--immediately after working up to a max snatch and clean and jerk--and my score was only about 30 points lower than my best score when I was doing CF fulltime.