Assuming you are fine with treating 1 GB as 109 bytes, not as 230 bytes, you could simply
1) pipe the results of FSUTIL
to the FIND
utility to filter out irrelevant rows,
2) use the modified command line in a FOR /F
loop to store the output into a variable,
3) cut off the last 9 characters from the variable's value and compare the rest to the required quantity.
And here's my attempt at putting the above steps together:
@ECHO OFF
SET required=60
FOR /F "tokens=2 delims=:" %%A IN ('fsutil volume diskfree c: ^| find "# of bytes"') DO SET "space=%%A"
IF %space:~0,-9% LSS %required% ECHO Space is less than %required% GB!
The above script processes the total space. If you want to check the free space, change "# of bytes"
to "# of free"
.