GNUpg command to encrypt/decrypt a file is working fine on InterSystems's Terminal but not working in the BPL process.

Terminal works under your OS user, BPL works under InterSystems user (typically irisusr).

This type of error is often caused by:

1. IRIS OS user does not have permissions to access the input and/or output file
2. IRIS OS user does not have required executables in PATH
3. IRIS OS user does not have required shared libraries in LD_LIBRARY_PATH

To solve issue (1) create input and output files in IRIS Temp directory - a path returned by:

write ##class(%File).NormalizeDirectory(##class(%SYS.System).TempDirectory())

To test for issues (2) and (3) add a $zf call which inits all your dependencies and returns version or some other trivia.

To adjust PATH and LD_LIBRARY_PATH use System Management Portal. Here's some info.

I recommend checking environment variables from inside your process with:

write $SYSTEM.Util.GetEnviron(Name)

For quick testing you can adjust environment variables for the current process with this community utiliy.