You mean training despite having had solid food? Actually once I caught on to the problem, which was very early on in training, I avoided doing it and so I never really had a chance to see how the reduced performance would have affected gains. Basically I switched to preworkout drinks early on. But of course due to circumstances from time to time I’d train having recently had a meal of solid food, and it’s been a consistent experience that I just didn’t do as well in the gym. But I couldn’t say from observation whether gains would have been less as a consequence.
In my case it’s not blood sugar, because actually for me, the more carbs the better I do in the gym, within reason. Certainly it varies between different people. Many have done extremely well with solid food as the only nutrition source, in some cases including before workout, and in some cases avoiding eating for some time before the workout. The thing to do is to see what works individually, which you are doing!