The Gist API provides up to one megabyte of content for each file in the gist. Each file returned for a gist through the API has a key called truncated. If truncated is true, the file is too large and only a portion of the contents were returned in content.
If you need the full contents of the file, you can make a GET request to the URL specified by raw_url. Be aware that for files larger than ten megabytes, you'll need to clone the gist via the URL provided by git_pull_url.
In addition to a specific file's contents being truncated, the entire files list may be truncated if the total number exceeds 300 files. If the top level truncated key is true, only the first 300 files have been returned in the files list. If you need to fetch all of the gist's files, you'll need to clone the gist via the URL provided by git_pull_url.