If there is no violation of copyright, you may add any presentation to your ResearchGate profile. I do not know whether it is possible to upload pptx files, but as a user I would prefer pdf. See also https://explore.researchgate.net/display/support/How+to+add+research for general instructions and also https://explore.researchgate.net/display/support/Copyright.
In my opinion, there is no limit to publishing your own presentations. The problem may occur when someone's copyright may be infringed. This may be important when, for example, the graphic elements used in the publication have been downloaded from some websites.
Adding a presentation is simple. Go to your profile, click the "⊕ Add research" button and select "Presentation" – from the menu on the right side of the website window. A dialog box will open where the file filter includes different formats.
You then need to decide whether you want the presentation to be public or private. You must also confirm your copyright to the posted content.
I am attaching a graphic with the current appearance of this menu…