Setting pixel color of 8-bit grayscale image using pointer

First of all you are copying too much data in this line:

memmove(data, grayImage.constBits(), size * sizeof(QRgb));

The size ob Qrgb is 4 bytes, but according to the documentation, the size of a Format_Grayscale8 pixel is only 8 bits or 1 byte. If you remove sizeof(QRgb) you should be copying the correct amount of bytes, assuming all the lines in the bitmap are consecutive (which, according to the documentation, they are not -- they are aligned to at minimum 32-bits, so you would have to account for that in size). The array data should not be of type Qrgb[size] but ucahr[size]. You can then modify data as you like. Finally, you will probably have to create a new QImage with one of the constructors that accept image bits as uchar and assign the new image to the old image:

auto newImage = QImage( data, image.width(), image.height(), QImage::Format_Grayscale8, ...);
grayImage = std::move( newImage );

But instead of copying image data, you could probably just modify grayImage directly by accessing its data through bits(), or even better, through scanLine(), maybe something like this:

int line, column;
auto pLine = grayImage.scanLine(line);
*(pLine + column) = uchar(grayValue);

EDIT:

According to scanLine documentation, the image is at least 32-bit aligned. So if your 8-bit grayScale image is 3 pixels wide, a new scan line will start every 4 bytes. If you have a 3x3 image, the total size of the memory required to hold the image pixels will be 12. The following code shows the required memory size:

int main() {
   auto image = QImage(3, 3, QImage::Format_Grayscale8);
   std::cout << image.bytesPerLine() * image.height() << "\n";
   return 0;
}

The fill method (setting all gray values to 0xC0) could be implemented like this:

   auto image = QImage(3, 3, QImage::Format_Grayscale8);
   uchar gray = 0xc0;
   for ( int i = 0; i < image.height(); ++i ) {
      auto pLine = image.scanLine( i );
      for ( int j = 0; j < image.width(); ++j )
         *pLine++ = gray;
   }