If you use basic html view, you will need to switch to "Standard View"** in order for the "gear" to show up, and go into the settings menu there. Clicking "Settings" while in basic html view mode gives a different set of (and more sparse) options, and the option for conversation view is not in there. This would not be *quite* so objectionable, but now you face the challenge of trying to get back to basic html view...
(a moment to digress...)
IMO gmail is one of the worst web experiences out there, and the difficulty in finding out how to change options and the inconsistencies involved with it is one example of that. And now the user faces another example of that; the challenge of trying to get it back to basic html view (I hate all the noise in Standard View). The only ways of which I know of is either to refresh the page, and be quick enough with the mouse to click the "basic view" link, which only appears for less than a second while the page is loading, or pasting
https://mail.google.com/mail/?ui=html&zy=h into the location bar*** (I've also read
http://mail.google.com/mail/h/ works too, but haven't tried it). Nice user interface, huh?
But back to the subject, what would be really nice would be a button on the page to toggle back and forth between the two. Each view has its advantages and disadvantages depending on the situation at hand. But if I had to be stuck with one or the other, it definitely would NOT be "conversation view".
** note 1: switching to "Standard View" from "Basic HTML View" is not as hard as going the other way -- just find the link at the bottom of the page.
*** note 2: after changing views, a link appears at the top of the page giving you the option to select one or the other as your default view. Do this now, as it won't appear again
