static static final Map<String,GTKStockIcon> is modified only in 'static' block. Then 'com.sun.java.swing.plaf.gtk.GTKStyle#get' method read from it withing synchronized (ICONS_MAP) block.
As this map is read-only and all its content is initialized in clinit we can safely remove 'synchronized' from 'get' call

        GTKStockIcon stockIcon = null;
        synchronized (ICONS_MAP) {
            stockIcon = ICONS_MAP.get(key);
        }