The GPIO pins are for digital data only. Commercial drum trigger modules use an A/D converter to convert the trigger output into digital data that the processor can manipulate. If you think, "That sounds simple enough," read on.
Low cost drum trigger modules multiplex several inputs into a single A/D converter, carefully designing for accurate timing across the multiplexed inputs when switching each input through to the A/D converter. Higher cost modules will use individual A/D converters for each input, making timing issues for A/D conversion moot, but driving up the overall cost of the system.
Once you have decided which method of conversion you want to use, then you have to decide if you want to have analog or digital trimming of the inputs (so your drum trigger signal doesn't go past the maximum the A/D can convert). Cheaper drum modules opt for manual trimmers (potentiometers controlling the input opamp gain) coupled with low resolution A/D converters (8 or 10 bit). Mid-cost modules might use automatic gain control (the processor controls the gain of the input opamps). Almost all drum trigger modules these days use high resolution A/D converters (16 or 20 bit) and use digital trimming (adjusting the gain after it's been converted to digital). The latter method consumes a lot more bandwidth because of the higher resolution.
When you get that sorted out, the next thing you need to consider is noise reduction. The triggers carry a whole lot of extra signal besides the main thwack, so you must filter that out. The simplest solution is an analog low pass filter on each input with adequate slope, but not so much as to cause ringing (18dB or 24dB/octave is good), and the cut off frequency not too low as to cause phase shift, affecting latency (100-200Hz cut off works well). The expensive modules do all this in software with a DSP or a very fast cpu. The Pi is not well suited for this kind of processing (for more than a few triggers), so an analog filter is your best bet.
Then you have to decide if you're going to use the Pi as an all-in-one (process the trigger signals, data manipulation, and sample output), or just as a data processor (processes trigger signals, data manipulation to convert into MIDI, then output MIDI). If an all-in-one, you will not have very much horsepower left over for handling input triggers after sending out audio. Keep in mind that even though the Pi certainly has more than enough capability to process triggers, it has much less capability to do so within 7ms or less (the approximate threshold most drummers start to detect latency). Divide that by the minimum amount of triggers you need for a simple 5 piece drum set (kick, snare, snare rim, hi tom, mid tom, floor tom, hi-hat, hi-hat pedal, crash, ride, ride bell) and all of a sudden the Pi's 700Mhz is now 700/7/11=9Mhz.
"9Mhz," you think. "Pshaw! I can do plenty in that." Well, you have to divide that by the sample accumulation rate for a trigger (use 1ms or less sampling rate to get really accurate peak detection within 2-3ms latency), plus divide by the size of the sample block you're going to be sending (the sampling rate chopped up by a rather loose 5ms latency for example) means that now you're right at the minimum of detectable latency (7ms). Because we don't want your drummer to feel like he's playing out of sync, the amount of processing time you have to detect a drum strike and send out a drum sample is now reduced to 1ms, for a total of 8ms processing time, which puts you just slightly over the average detectable latency threshold.
If all this seems confusing, start out very simply. Ignore the GPIO pins. Those are for manipulating digital signals, not audio. Instead, buy a cheap USB audio adapter. Read through the threads here on how to get that to work on the Pi. Once you can record and playback simultaneously with any available audio application, learn how to do something simple like program an application that you switch left and right channels with. Once you know how to manipulate the audio data like that, you can start experimenting with reading your drum triggers that you have plugged into the USB adapter inputs, and then sending out a drum sample at the output. Of course you're limited to 2 triggers, one each for left and right input, but it's enough to get your feet wet.