Octagon boxes are gangable. Round multigang boxes are basically a rectangle witha semi-circle at both ends.
Drywall boxes are round, orange or red (not grey or black like the other box types), and a bit smaller than masonry bosex. They have 1cm long mounting strips on the sides. When the box is inserted into the wall the strips are turned in behind the back of the box. When the screws are tightened they turn out 90 degrees and clamp behing the drywall. The box is kept from disappearing inside the wal by a 2mm wide plastic flange around the box. The holes need to be very exact and are usually cut with a hole saw for the drill. I'm always afraid of tearing out the box with a nice section of drywall when pulling a plug out of such a receptacle.

[Linked Image from elektrofachmarkt-online.de]

drywall box

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octagon gangable box (only available with conduit knockouts, a strain relief isn't considered necessary since the cable is held in place by the surrounding plaster anyway.