You get this short gap, because the new amp first has to be loaded. So there's no way to get this without a gap. Probably the amp block also does a quick fade-out and fade-in when changing amps, so you get no bad noises between the ampsounds.
If you use scenes, you can run two amps simultaneously in parallel(!) and use scenes to switch between bypassed states. E.g. on scene 1 amp1 isn't bypassed, but amp2 is. That way you hear amp1. Amp2 is running, but doesn't get a signal (or gets the signal, but doesn't output anything). On scene 2 you could do the opposite and bypass amp1, but don't bypass amp2. When switching from scene 1 to scene 2 amp2 will instantly receive a signal / output the signal, which happens instantly. Amp1 on the other hand instantly doesn't get a signal anymore / doesn't output a signal anymore.
This works, since no new amps need to be loaded when switching.