Você precisa atualizar as listas do repositório apt usando:
sudo apt-get update
tente novamente
sudo apt-get install gparted
Você também pode instalar o Gparted da fonte. Para fazer isso, execute os seguintes comandos no Terminal:
sudo apt-get build-dep gparted
sudo apt-get install git gnome-common
git clone git://git.gnome.org/gparted
cd gparted
./autogen.sh
make
sudo make install
Além disso, se nenhum acima funcionar, por favor, verifique o seu /etc/apt/sources.list ou tente mudar para o servidor principal e execute novamente: sudo apt update
lista de fontes padrão do apt:
# deb cdrom:[Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1)]/ xenial main restricted
deb http://archive.ubuntu.com/ubuntu xenial main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu xenial main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu xenial-updates main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu xenial-updates main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu xenial-backports main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu xenial-backports main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu xenial-security main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu xenial-security main restricted universe multiverse
# deb http://archive.ubuntu.com/ubuntu xenial-proposed main restricted universe multiverse
deb http://archive.canonical.com/ubuntu xenial partner
deb-src http://archive.canonical.com/ubuntu xenial partner