I've been doing a lot of this lately and it's very simple. Open the .gig file you wish to work on into Gigastudio Editor. Right-click in the column where the sample folders are listed and select "Wave Pool Convert" from the menu. You will see under "Convert stereo to mono" three options - Combine L+R, Keep left channel and Keep right channel. Note: this command affects all samples in the entire .gig file. If you want different forms of mono from the samples in your .gig file, you'll have to separate them out into another file (just do a "Save As" with another name).
To decide which mono you prefer, launch Gigastudio and load the instrument you wish to hear. Click the mixing board icon. On the stereo channel that your sound is coming through (probably 1&2) adjust the "Width" knob to "center" to hear L&R summed to mono. To hear left or right by themselves, click the "Link" button at the bottom of the fader to unlink the fader into two mono faders. Pull L & R down alternately to hear the difference.
ok that really helps. but from the experience of everyone out there, what is better? Combine L+R, or keeping one? i really have a lot of sample libraries and if it's possible to avoid testing each one i will take the advice of anyone who knows.
If the original was recorded using a mid-side technique, you can combine the two channels for a perfect, centered mono recording. If the original recording used and X-Y or a spread-mic technique, you may get phasing problems if you combine.
I'd recommend a single channel approach, given that you will batch everything. As long as you don't destroy your originals, it's a good way to go. If any of your sample libs ends up quieter than before, you can compare it to the original on a case-by-case basis. There's the chance that a given lib was recorded near the right mic, but you did the conversion from the left channel.