If your article is OPEN ACCESS or CREATIVE COMMONS, it is permissible to upload it for public access. Otherwise, we need to wait for a couple of years before we can upload other types of journal articles. However, ResearchGate has a mechanism that allows you to send your papers privately through the platform without showing them publicly.
In many cases, you are not allowed to upload full texts, even not after some years. Some publishers do not allow to upload even preprints. You have to check individually in each case by consulting the website of the publisher or journal. See also https://help.researchgate.net/hc/en-us/articles/14846037644817-Copyright-and-ResearchGate.
Dear Loren Latta If you received these request from RG then they mean: upload them as "Private full-text". This way you can send the full-text to those who request this here on RG Although Wolfgang R. Dick is absolutely right that some publishers keep your full-text behind a paywall forever (and can thus not be made publicly accesible) it is completely legit to send the full-text on a one-to-one basis.