I am try to use el-get to manage my packages because it is good to update the latest from github. But sometimes the package I want is only available in ELPA. So I use
(:name <package_name> :type elpa)
to install them.
Now my problem is: I installed org-mode with el-get and try to install org-gcal by
(:name org-gcal :type elpa)
The "org-gcal" depends on org-mode 8.2.4 which I already have under el-get directory, but it still said it can't find it and will install org from elpa again.
How could I solve this kind of dependence problem between elpa and el-get?
You can't “solve” this dependency problem in that you could teach ELPA about el-get packages. That's not (easily) possible.
el-get understands ELPA packages, but not vice versa. ELPA does not see el-get packages, and hence cannot use them to resolve package dependencies.
The only reasonable way out is to use ELPA to install Org, and other packages as well. Recent Org releases are available from the default GNU ELPA archive or from Org Mode's package archive. The popular MELPA archive provides VCS snapshots for over 1000 other packages.