The first step would probably be to convert the samples into AIFF files, that's what GB's sample files are in.
But so far that's only a collection of samples - what you need is a sampler file that tells the software which sample to trigger with which key (the .exs files in GB's instrument library).
Since your software is old, t works pretty well with soundfonts (the latest versions have problems), so you might try this software to create them:
All about soundfonts:
P.S.: The softare costs $60, but you can create 10 instruments for free.