First off, let's address the question, what is gated content? Gated content typically refers to articles and or sources that require a login to access. Reasons for this could be you pay for a subscription, are a member, etc.
For example, the New York Times, Wall Street Journal, and The Economist all have paywalls that require you to pay for a subscription to read their articles or read their articles after viewing a certain number of articles for free.
Another example is an association or organization that produces a publication(s) for their members and requires your member login to view it.
Should you use "gated" content?
This really depends on what you are comfortable with as the publisher of your newsletter. Likely your audience will not receive more than 1-2 articles from a publication that has a paywall within your newsletter therefore they may be able to click in and use their free read of the day for that article. Also, many of your audience members may already be paying for subscriptions to some of your selected sources.
For articles coming from membership-based publications, this may be more personal to your audience. If you are publishing a newsletter that is related to a particular organization and your subscribers are all members then you shouldn't run into any issues. If you are publishing to a wider array of subscribers then you may not want to include sources that require membership log in.
If you have any questions about whether a source requires a log in or if you should include it in your newsletter, please email firstname.lastname@example.org!