This option is much slower than driving your own car, but it is an available and inexpensive option.
It is best to find a bus to take you there. There are different options and it is not difficult to find them.
Another option is to take the tram L1 to Benidorm and from there the tram L9 to Garganes. This is the option that will take you the longest, about 2 hours according to google.