I'm not sure how you've set this up within Supercollider, but I do know that "good" settings for a gate are completely signal-dependent. So you're probably going to have to connect your gate arguments to gui sliders and "wiggle 'em" to find appropriate settings. That is how you'd set up a hardware gate. First set up the 'threshold' so the gate briefly crackles on and off for your desired signal, then adjust the 'clamptime' and 'relaxtime' until it sounds natural.
Generally, because of the huge difference in volume and frequency between different drums, it's very difficult gating a whole drum-kit. I would suggest "splitting" your signal into different frequency "brackets" (one for each drum), and gating each one separately.
For the feedback problem, to state the obvious, try to set the mic up so it picks up a bare minimum of the speaker - point it 180º from the speaker, if they have EQ, try to attenuate the frequency that feeds back, or ideally, turn the speakers off and use headphones.
Sorry for not posting code - no time today. Hope I've been some help.