The Libgcrypt documentation doesn't mention the salt-length parameter for RSA-PSS padding. Its support apparently has been added in 2016 but is still not documented at all. Moreover, for some strange reason it requires the value to be specified as a decimal string, like (salt-length "255"), and doesn't understand, for example, hexadecimal constants, like (salt-length #FF#), quietly treating them as 0. All this cannot be understood without digging into the source code...