Gbt3fC79ZmMEFUFJ is certainly a poor choice of password now, seeing that your question has generated a lot of activity on Stack Exchange and has thus widely published the password, I don't think there is any discernible reason we can pin down for why this website would say it's bad in the first place, because, quite frankly, it is not a credible source.
The source that you're drawing from appears to be a loosely organized list of security tips of low to middling quality. Some of the tips are good (e.g. use 2-step authentication whenever possible, lock your computer and mobile phone when you leave them), but some of them have been questioned by security experts (e.g. change your passwords every 10 weeks, use at least one uppercase/lowercase/number/symbol) and quite a few seem downright paranoid (e.g. tip 22, which states that if you have a WiFi router in your home, nearby attackers can detect what passwords you're typing in because of changes in the WiFi signal from moving your hands, and that using an on-screen keyboard with random layouts will mitigate this).
And while it is probably not an issue that most of the tips are poorly proofread (with many spelling errors and punctuation inconsistencies), it is an issue that absolutely none of the tips are cited, and that the whole website is anonymous. There are no quotes from security experts, no articles that we can read up on for further information or discussion. And the website contains absolutely no author information - nowhere on the website, or even in the website's Whois registration, is there any way to contact the author or even know who they are.
This essentially means that we have little to no way of determining how good of a tip "
Gbt3fC79ZmMEFUFJ is a weak password" is, or what it even means. We certainly have no way to contact the author of this website and ask them ourselves, and I actually doubt that it would even be useful to do so. I think that we are best off ignoring this suggestion, taking way only the fact that
Gbt3fC79ZmMEFUFJ should not be used because we have talked about it at such length.
TL;DR: Passwordsgenerator.net is just another anonymous stranger on the Internet - just ignore it.
As an addendum, you may be interested in some other things I found when I was inspecting the website itself, to get better context on what it was and perhaps find more information on it. What I found wasn't very pleasant:
- The website provides an option (default checked, but still) to not generate the password on the server. I did look at the source code and the network traffic view in the Dev Tools window, and it does, in fact, honor this request, but there's no way to tell that it actually does that without reading it.
- The generator users
Math.random() is a cryptographically insecure random number generator, making the generated passwords surprisingly predictable and actually far fewer in number than you'd be led to believe - for instance, if your implementation of
Math.random() has only 32 bits of state, then regardless of what settings you use, only 4 billion unique passwords can be generated, which means that any number of characters beyond 7 or so is meaningless. A true random password generator should instead use the
window.crypto.getRandomValues() call instead, which sources every single bit of the password from a true entropy source somewhere in the computer.
- The password generator is also placed on the page strangely - the code for the generator itself is minimized, but the code for configuring it is not. It almost appears as if the website author copied the minimized password module from another source, such as another website.
- The website also tries to use Google Ads for some odd reason. It doesn't actually invoke them - it just has a signature at the bottom for adding the Google Ads ID to the
window object - but any presence of actual ads will also potentially compromise any data entered into the page, even if the page is served over HTTPS.