I think what you want is currently not implemented in the aspect-gui. You can use it this way:
if you want to create (save) a new .prm file, or this way:
if you want to modify and save an existing .prm file. You can currently not open an existing .prm file directly from the program, and it would be very complicated to implement that. The reasons are complex, if you want to work on this, I can give you further advice, but the two examples that I wrote above should allow you to do everything you need (create a new .prm file, or modify an existing .prm file). You just need to close the aspect-gui and reopen it, if you want to read a new file.
@cgw0814: You are right in your last comment, every default parameter is not written in the .prm file. This is on purpose, because if the parameter is not in the file, ASPECT will use its default value. I.e. there is no reason for you to write
Use year in output instead of seconds = true
because ASPECT will assume it anyway if you do not set it to
false. If we would write all default values, the .prm file would have hundreds of lines.