How do I open a .gid file? I have some files that have .gid extension. What Mac software should I use to open it?

I use OS X 10.5.8.
It could be a Windows global index file (.gid) or something else completely different depending on where it originated from. If indeed it is the common Windows type, it's used basically tell Microsoft help documents where to position the window on the screen, not much more. 


If you want to open it
Control mouse-click > Open with... > Text edit


It might be gibberish or it might not; either way you could take a look if you're completely curious. Sometimes unknowingly, mistakenly, or haphazardly, software developers will use .(lll) whatever in relation to the software they've created while another application or computing platform also uses that extension; this can result in confusion, problems and conflicts.

Take a look at this document which lists the most commonly used extensions.