code: protected void registerGridGroupAttributes() {
          addAttributeProcessor(new GridSpacingLmlAttribute(), "spacing");
          addAttributeProcessor(new ItemHeightLmlAttribute(), "itemHeight");
          addAttributeProcessor(new ItemSizeLmlAttribute(), "itemSize");
          addAttributeProcessor(new ItemWidthLmlAttribute(), "itemWidth");
          // FixedSizeGridGroup:
          addAttributeProcessor(new BlockIndexesLmlAttribute(), "blockIndexes");
          addAttributeProcessor(new ItemsAmountLmlAttribute(), "itemsAmount");
      }
docs: GridGroup attributes.