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 Sourcetree has a lot to offer and, like many developer tools, finding and using it all can be a challenge, especially for a new user. Everyone might not love ...
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