This seems to be server related, since so many people are facing the same problem. Maybe an instance of the download server is down and only the loadbalancer will make it eventually work?
On the download page (e.g. https://packages.gitlab.com/gitlab/gitlab-ce/packages/ubuntu/trusty/gitlab-ce_7.11.4~omnibus-1_amd64.deb), you can find the instructions but also a download button (look upper right), which you can wget:
wget -c https://packages.gitlab.com/gitlab/gitlab-ce/packages/ubuntu/trusty/gitlab-ce_7.11.4~omnibus-1_amd64.deb/download -O gitlab-ce_7.11.4.deb
sudo dpkg -i gitlab-ce_7.11.4.deb
An alternative approach if you really want to use apt-get would be, to automatically try again (e.g. in a screen)
while true; do apt-get install gitlab-ce -y; sleep 303; done
and check if it eventually worked