Sexing the dempsey is rather simple. Males will be larger and have longer more pointed fins with very little blue on the gill cover if any and will be in dots if its present whereas the female will have splotchs on her gill cover and have less color throughout the body. The female also tends to be more "squat" in shape.
The larger one is clearly male. The smaller one is probably male as well but a little difficult to tell.
I used to have GTs with many CA/SA cichlids and never had them cross breed.