You will need to retrieve the files of your articles from your computer or the internet. Then you can upload them to RG in PDF format. ResearchGate does not do this for you, although they do find articles that are published in journals and ask you to confirm your authorship, include it in your publications list on RG, and ask that you upload it if you have the copyright. If you do not have the copyright, having relinquished it to the journal publishing your paper, you can upload the article to RG in the private section. This enables you to easily send the file to those that ask you for a copy of the article through RG.
Thank you, Prof Raymond for your answer to my question, however, I authorise and urge RG to make a google search of most of my articles on the internet and upload them as they did already in some. I have no objection.