There is no direct way to do what you want in PGP Command Line. The output file will either be the "Suggested file name" (which should match the original file name before encryption), or the name you specify using the "--output" (or "-o") option.
To do what you want, then, you'll have to construct the appropriate output file name, and pass it as the "-o" option. E.g., use a script like this:
#!/bin/sh
# call as "scriptname input-file passphrase"
# output file will be the input file name with ".pgp" removed
pgp --decrypt $1 -o `echo $1 | sed s/.pgp//` --passphrase $2
Obviously you'll construct your own script based upon your own environment and usage.
Regards,