OSDev.org

does it really matter if the accessed bit is set to 1 when i set up my GDT?

no. that's a write-only bit for the cpu. It will never read or use its value. It just fills it with a 1 when it accesses the segment so that the software can keep track of segment useage and optionnally implement segments-based virtual memory (swapping out a whole code segment from memory when it's no longer used).

no need to say that paging-based virtual memory has been preferred by OS designers.

what does the available for system software bit do?

Nothing. It's available for use by system software (hence the name). You can use it for whatever you want.