Over, "modulo" means taking the remainder of a division.
1 modulo 2 is 1 ( 1/2 = 0 remainder 1/2 )
2 modulo 2 is 0 ( 2/2 = 1 remainder 0/2 )
3 modulo 2 is 1 ( 3/2 = 2/2 +1/2 = 1 remainder 1/2 )
So what he's suggesting is that the installer sees 60 GB free modulo 2 = 0 GB. His bugfix involves adding roughly 1GB of files to your drive, so that the free space decreases to 59 GB. 59 modulo 2 = 1. Thus, the installer sees 1 GB of free space and happily proceeds with the installation.
Dunno if that works, but give it a go. It's worth a try, I say.

Downloaded again, no download managers, It installed well now. :)
Strange. Never had a problem downloading with download managers.

Chances are the installers used a 32-bit signed integer to hold the free disk space. Not coincidentally, 2G is 2^31.
I know for a fact that the trick of adding junk files worked for my original copy of Fallout 1 two or three days ago.
If GOG has the source code to the games, it would be well worth fixing these issues.