Any chance of a picture? It might be easier to figure out the brown spots and what to do with them.
The Gasterias I grow tend to be a little sensitive to sun this time of year and they react a couple of different ways. The whole leaf may turn brownish or reddish, like overcast on the regular colors. Or small black spots may appear on the leaves with the greatest exposure to the sun. The first kind of reaction is entirely transient, the second is permanent (until the plant grows new leaves). I grow my Gasterias in the sun though, and you should not see those reactions indoors. Except maybe if the plant goes from some dark position behind the curtains right into day-long sun, maybe. Abrupt transitions in exposure are where the risk occurs. Indoor sun is not direct because regular window glass cuts quite a bit of the UV, making the light that passes through kinder to plants.
