You won't lose time since every gcd is 2.5s (or around 2.4 at medium skill speed at lv 60), you can press a gcd, switch and press the second gcd maybe 0.5-1s before your actual gcd cycle is complete, and that 2nd gcd will come out exactly when the 1st gcd completes, that's what people meant by "queuing gcds."

Yes of course it's better to test it yourself to see if you're happy with the performance of macros. Personally I probably haven't spent as much time as you when it comes to testing macros so take my comments with a grain of salt, like what the other person posted earlier.