You have a choice between merging and rebasing, this blog post might be useful: http://blog.sourcetreeapp.com/2012/08/21/merge-or-rebase/
Usually, you'll want to merge if these are different branches and they're both visible on GitHub already. In SourceTree, you can use the Merge toolbar button to merge another branch into your current branch (so if you have master checked out, you could merge gh-pages into it), or you can right-click on a branch in either the sidebar or in the log and select 'Merge' to bring that branches changes into your current branch.
Supported Platforms macOS Windows To make using Sourcetree as simple yet powerful as possible we embed (bundle) dependencies such as Git, Git LFS, and Mercurial. We strive to keep these...
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