The new gutter installation is faulty, or clogged; any what way, there is a major leak on top that lets water flow down the wall. Looking at the damage, it is possible that it also comes onto the top or even down the inside of the brick wall. That a lot of water flows where it shouldn't is evident by the dark patch on the exterior wall running all the way from the gutter to the bottom of the image. That is the root cause and needs to be fixed asap because, as is obvious, the water seeps through into the interior, ruining interior walling and decoration and causing a mold issue. Over time, it also compromises the wall structure.
If there is interior insulation and drywall, that needs to be removed and discarded. Chances are that the water has flown much farther downwards, undetected, behind a drywall. If there is only solid plastering, it depends whether it has become brittle and soft. If so, it needs to be removed and replaced.
In any case, the mold is a serious health issue and must be dealt with immediately. A first aid measure is to use a chlorine solution. Many anti-mold fluids from home improvement stores are actually nothing but that, which can be easily self-made from chlorine bleach sold in those blue bottles available in drug stores, mixed with water. This arbitrary page has some good recommendations, except that I'd still use it on drywall as well; not as a permanent solution but until it can be removed. The main thing to observe is that chlorine itself is a health hazard as well — ventilate well so that you don't drive out the devil with the Beelzebub. Perhaps, especially if this is a sleep room or if one inhabitant has asthma or similar, consider using hydrogen peroxide. I also would not spray hydrogen peroxide or chlorine, even if that is the recommended way to apply it on those websites (what are they thinking?). Simply sponge it on, with rubber gloves, and only as a first aid.
