I'm going to go ahead and post this because there seems to be a problem that nobody is addressing but one poor soul mentioned (that I could find). This is only in gmail.
I too have an APPLE keyboard. The APPLE keyboard does not sport an "insert" key--at least, not one that is labeled. The insert text feature (as opposed to overwrite) seems to be shut off rather randomly in gmail, and without an insert key, I am at the mercy of it being randomly turned back on.
I thought I found the solution, because I used control-caps lock and it started inserting again, but could not turn it back on. However, when I used the "home" key, overwrite did turn back on, but -- wait for it -- I cannot turn it off again. So, as I said, apparently completely random.
I have tried all the F# keys, and all the combos I can think of with control, apple and alt/option keys. The simplest solution would be if there were say, an easily accessible tools and options section on Google. Yeah, I can dream. So far, this board is as far as I can get.
Finally, I have been able to work around this by putting in a "return" before where I want to add text, adding the text at the end of the line and then removing the return with a backspace.
If someone has solved this with an Apple keyboard and/or knows how to find the tools in Chrome, maybe they can post here. Thanks so much.