In a VCD file I would like to specify that some events occurred at a particular time. To that end, I tried to define a single bit signal which value is almost the time 0, and switching this value to 0 and then back to 1 in the same time (at the time when my event occurs). Unfortunately nothing is displayed when looking into the file with gtkwave. How could I achieve this behavior ?
Use event as your variable data type and use -> to force an event on it. It will show up in gtkwave as an impulse arrow in zero time. No tricks with timescales are necessary.