The answer depends on the operating system you are using.
If you are using the standard Linux from https://communities.intel.com/docs/DOC-22226, there isn't an official repository.
There is an unofficial one run by AlexT and you can set up your system to access it, but it is easier to just go ahead and install his image that includes development tools and is setup to access his repository. Go to http://alextgalileo.altervista.org/blog/updated-galileo-devtools-image/
If you are using XDK IoT it has a repository and you can use opkg. see Galileo Gen 2 with IoT devkit problem
If you are using one of the Debian or other versions of Linux, you should be able to use apt-get to install applications, but there is no official Intel repository.
Hope this helps
I'm using linux image compiled by bitbake,I just want to install nfs-common through opkg .
but i 'cant find one in the repo .
Since you rolled your own, I don't know if this will work, but you can look at what AlexT's instructions for adding his repo at Galileo Package Repo Configuration Instructions and see if it matches what you built. Otherwise I would guess you need to download the source code and compile it, assuming you included the development tools.
Sorry can't be more helpful, but i just use the standard versions.