Is depositing a document already available online in ResearchGate a problem?
I don't think your question is (or was) unclear, but there are a few things you don't seem to consider.

First, each author has a say in a) the final form of the document to be published, b) when to make it public, and c) where to publish it. You seem to think that c) doesn't matter. While my opinion is that ResearchGate generally is pretty benign, your advisor might not agree. However, we don't need to debate the relative merits of ResearchGate to address this question. Suppose instead that your coauthor were to submit your piece of writing to a site you fundamentally disagree with, or one that's likely to reflect badly on your work by association (e.g. a site with a reputation for hosting crank or crackpot papers). Yes, you would be justified in being angry at your coauthor in this situation. To avoid such troubles, it's always best to discuss with your coauthors where, when, and how you submit something before doing so.

Second, when something gets published in a journal you usually sign a copyright transfer agreement. Depending on the journal, you might be allowed to host a copy of the published paper on your personal website but not third-party sites like ResearchGate. In that situation you might actually sabotage getting it published. Now, in this case you say the document is a grant proposal rather than a paper. I don't know if the same applies there or not, or if your advisor might simply want to keep the visibility low on this document, and instead focus on giving publicity to actual peer-reviewed publications later. After all, such documents might contain ideas that didn't work out, or ideas that a competing research group might "steal" if they were to see it online.