Automated way

So, the very automated way:

git clone git@github.com:mgor/docker-ubuntu-i3-gaps-builder.git
cd docker-ubuntu-i3-gaps-builder/
make

Packages available in packages/.

Build environment

First, get the build environment and start it:

git clone git@github.com:mgor/docker-ubuntu-pkg-builder.git
cd docker-ubuntu-pkg-builder
make

Dependencies

Install the needed dependencies:

apt update
apt install libxcb1-dev libxcb-keysyms1-dev \
libpango1.0-dev libxcb-util0-dev libxcb-icccm4-dev \
libyajl-dev libstartup-notification0-dev \
libxcb-randr0-dev libev-dev libxcb-cursor-dev \
libxcb-xinerama0-dev libxcb-xkb-dev libxkbcommon-dev \
libxkbcommon-x11-dev
apt-get build-dep i3

Build

Get i3-gaps from github[0].

git clone https://www.github.com/Airblader/i3 i3-gaps
cd i3-gaps

If you want to run on the stable branch:

git checkout gaps
git pull

Build the packages:

debuild -i -us -uc -b

If successful, the packages will be in ../. Transfer them to your host and install.