In my 15 or so years writing as a skeptic I've investigated a number of fringe theories and observed the approach taken by their proponents. A number of similarities have become apparent from theory to theory, and two of them are especially relevant to this thread.
First, conspiracy theorists grossly overstate the objective credibility of their claims. They are inclined to believe their theory is more credible to the general public (or even to subject-matter experts) than it is. On the one hand I believe this is due partly to the isolation that conspiracy theorists generally build around themselves, preferring to associate mostly only with their true believers. On the other hand I believe it is also due to their misconception and/or rejection of legitimate authority, which gives conspiracy theorists an underestimation of how confidently subject-matter experts are able to reject their claims on their face.
Second, conspiracy theorists presume that the only reason their theory is not widely accepted is because it is not widely known. That is, conspiracy theorists generally avoid the notion that there can be a well-reasoned disputation of their claims. Hence they tend to attribute disbelief among knowledgeable parties as ignorance of the claims and/or of the supporting sciences. They argue that affirmative agreement would be much more prevalent if the theories were more widely disseminated, and analogously that any one person who disagrees with the theory must do so only because he is uninformed about it.
These significantly address the degree to which Gage's claims are viewed and considered.
Your second point hits at human nature generally.
"If you only understood the issue the way I understand it, then of course you would agree with me."
I believe this is driven by a psychology that is very hard to overcome:
1) I am a reasonable person. (Ego)
2) I have examined the matter to my satisfaction and come to this conclusion.
3) Therefore, anyone else who is reasonable will do the same if they get the same information.