First, you must make sure your soundcard has multiclient drivers (most pro-audio soundcards do).

Go into Logic, Audio, Audio Hardware and Drivers, Audiodriver2, and under ASIO, change the MAX I/O Streams to be two less than the outputs you have on your card. Say, if you have a MOTU 828 (which has 18 outputs) and you want to use analog 7-8 for Gigastudio, select 6 I/O streams in Logic. Gigasampler must use the LAST group of I/O because Logic must use consecutive I/O streams, staring with the first.

In Gigasampler/Studio, go to Settings, Hardware/Routing, make sure your audio card's drivers are in the top box, and choose the last pair of Outputs Enabled. Now the two programs will not interfere with their I/O.