Since Google has gone to shit, maybe you haven't found the following sites:

https://www.hamburgergitarrenforum.de/ https://gitarrehamburg.de/

The first one event mentions regular meetings but not whether they are still happening. I would suggest reaching out to the Vorstand and asking if they know where to look for a jam, they excplicitly write that they see themselves as a networking platform for guitarists in Hamburg.

And as a German I would like to echo what others said about clubs in germany: I have met half of my friendgroup and my long-term partner in a Verein. Nearly every person I know is or was involved in one. Its not always easy to find the right one for you, but I believe it is the most straightforward way to expand ones social circle in Germany.