For the past couple of days I haven't been able to attach or send attachments of any sort (eg, .jpg, .gif) via Gmail.
Size shouldn't be a problem; all of attempted attachments are below 400KB.
Whenever I try to attach something, it appears to proceed, only to "lock up" by going into an interminable loop or just "treading water" and finally stopping.
In one instance the e-message with gif attachment was successfully sent (found in my Sent Mail folder) but according to the Compose Mail page it wasn't. Go figure.
According to the Help site, a known issue of that ilk has been fixed.
Guess what? Not.
Any thoughts as how to address the problem?

That sounds like Google's problem to solve, not yours. Report it so they know their "fix" was not successful and you are still encountering the problem.

I tried that, but the report form on their website requires information that doesn't exist (and that therefore I can't provide) and therefore the form cannot be submitted. There is no other means for submitting reports. That's why I decided to give it a shot here.
Checkmate.

First I should explain my GMail account belongs to a specific group, so my experience may well be different from yours. I had never attempted to report a GMail problem before and I was surprised at just how difficult it was to find a place to do so is. The only place I found was on the GMail Forums, specifically selecting Report an Issue. Apparently you have to join a group to do so, but that is not too onerous. After that it is pretty much a forum like this one but I did not encounter any forms or questions that required answering other than describing the situation just as you did here.

You might give that a try and see if you get any resolution to your problem there. Or you might just wait and the problem will probably simply go away without you ever knowing why or how.

Yes, one pretty much has to luck onto the location to report something. Google is relatively secretive about such things.

Lacking any other solution, I'm going to opt for your latter approach.

just as a test, perhaps try a different browser?

I would if I had any (including Safari) which I could make work to my satisfaction. But I don't.

I just tried it again (for the first time since I reported the issue), and now it works.