You need to check the requirements of each journal. Some allow you to share "pre-press" versions. Others say no to any sharing on open sites such as ResearchGate.
For subscription-based journals it is indeed officially not allowed to share public full-text versions of your publications (although some have an embargo period and after that you are). As indicated some allow pre-print versions.
However you can always upload the full-text in the "private" mode. If people want to read a paper of yours they can request a full-text (and you are always allowed to send them one on a one-to-one base).
You need to check if the Journal allows you share the paper on open sites such as ResearchGate. If you can not share it as an open paper you can upload the private full-text and share the abstract.
You can find instructions on copyright below https://explore.researchgate.net/display/support/Copyright
ResearchGate lets you:
1) Upload public copies of your full-texts
2) Store private copies of your full-texts that are accessible to you and your co-authors
3) Share private copies of your full-texts with others
It is possible to post entire articles and chapters of monographs on the Research Gate portal in the open access formula without violating the rights of the journal and the editing of the monograph, if the copyrights belong to the author of a specific published scientific text and / or the editors of a scientific journal or the editing of a monograph allow such additional dissemination of the published a scientific work by its author or authors.
There's no way to give your articles to ResearchGate without disturbing Journal rights. If you want to do so, be bold and upload a pdf. version to ResearchGate and forget about the consequences.
If your articles are published in the open-access, you can fully share them anywhere at any time. On the contrary, if your articles are published in the traditional way, you just only can share their preprint type/accepted version (anywhere at any time or after an embargo period, depending on the policy of the publisher).