For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ykoba:ruby/openSUSE_Tumbleweed/home:ykoba:ruby.repo
zypper refresh
zypper install rubygem-gdk_pixbuf2
For openSUSE Leap 42.3 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ykoba:ruby/openSUSE_Leap_42.3/home:ykoba:ruby.repo
zypper refresh
zypper install rubygem-gdk_pixbuf2
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ykoba:ruby/15.4/home:ykoba:ruby.repo
zypper refresh
zypper install rubygem-gdk_pixbuf2