how can I get purgeable space as shown in Disk Utility's GUI 30.41 GB (2.05 GB purgeable) from
diskutil CLI ?
Unfortunately, this is not possible.
The purgeable space reported by the (GUI) Disk Utility is a summation of the temp files, caches, backup files, Trash, and even some iCloud data (from Optimize my Mac). A simple reboot will clear out a lot of these as will emptying the Trash.
You could potentially get an approximation of this info with the command
% du -ch -d0 ~/.Trash ~/Library/Caches $TMPDIR
That will get you a nice summation of the bulk of the purgeable space excluding iCloud optimizations and any other temp/cache files not stored in the directory referenced by