Go to Terminal Terrestre Quitumbe. From there you take a bus to Macas. They depart regularly throughout the day and it takes about 8 hours. From Macas you take a bus to Gualaquiza which takes about 5 hours.
From the bus terminal, take a bus to Cuenca via Cajas National Park (Parque Nacional Cajas on the map). It is important to remember that particular route, as there is an alternative way that goes through Cañar that takes longer! The shorter route (through Cajas) is about 4 hours to Cuenca. Buses from Cuenca leave regularly for Gualaquiza and also take about 4 hours.
Buses from Cuenca leave regularly for Gualaquiza and take about 4 hours.
Buses from Loja leave regularly for Gualaquiza and take about 5 hours.
*Note: If you are arriving from the southern direction (such as from Piura, Peru, for example), travelling through Loja from there is advisable. Be aware that Loja Internacional is the only company that offers cross border service. Crossing the border from Peru involves the bus dropping off passengers with immigration in Peru to check out, and then waiting for all to walk the short distance across the bridge to check in on the Ecuador side before proceeding towards final destinations. You may want to stop along the way for a night to rest, but it is feasible to take this one bus line (Loja Internacional) complete from Piura, Peru (with multiple short stops) to Gualaquiza, Ecuador.