Citable code snippets as gists
While not in the field, I see two options.  Even if none of them is giving a doi, either:

A) Consider runmycode.org as a deposit for your code. It is backed for example by the French CNRS, the nation's largest body of public research, the Alfred Sloan Foundation, and Elsevier as a larger publisher. One major axis of action actually is to preserve code used in journals.  The service is for free, and all languages are permitted.  Consequently, Elsevier's Journal of Computational Science encourages authors to do so:


  You can enrich your online article by uploading relevant computer code and data to the RunMyCode repository. Once published, your article on ScienceDirect will be linked to a dedicated RunMyCode companion website via the "Data for this article" application displayed next to the article, in the right hand side panel. This linkage will allow readers to access your code and data via the RunMyCode companion website. To create a companion website, please go to: http://www.runmycode.org/home


(source, accessed on June 30, 2017)

B) Secondly, lesser for the intended publication in a journal than for broader re-use of your coding by others, there is a dedicated CRAN website here to submit R packages to their servers.  Obviously, to be checked with the policies of the targeted journal.