Corrigindo "Erro GPG"

Remova as listas antigas com:
# rm /var/lib/apt/lists/*

Atualize as chaves:
Exemplo:

# cd /root
# wget http://www.webmin.com/jcameron-key.asc
# apt-key add jcameron-key.asc
E atulize as listas e o sistema:

# apt-get update
# apt-get upgrade