Where do we draw the line between a company "abusing" its network effects and a company avoiding being abused by other companies? (e.g. Imagine a app store that distributes only free software backed by a big corp, then a small proprietary app maker complains that they don't want to accept their app because its proprietary).

Is the size of the company the criteria that decides if they are in the right or in the wrong?

I don't think the app description of Gab is an appropriate content. Google seems to assume that the content of the Gab app will be innapropriate. Given this is about freedom of speech I'm not sure they should be the one deciding this. Are they also removing Gab from their Google Search results?

Yes. That's the whole point of antitrust law.

They ran atop the same network that everyone else had access to and achieved, the accurate analog to the railway example is the ISPs and not these companies.