Orio would be a good intermediate point to take public transport to Gernika. Zarautz too but may it's too close to Gernika. Lurraldebus link both Zarautz and Orio with Gernika. For info about schedules:
www.euskotren.eus/es/Autobus/Lurraldebus (Click on Urola Kosta)
Munitibar is an option. Elexalde is another one but not on weekends (not sure about public holidays). Bizkaibus links both towns with Gernika. The Bizkaibus web isn't easy to navigate but if you want to give it a try, you'll find a link at:
www.euskotren.eus/es/Autobus/Bizkaibus (click on the link with 3 w)
Portugalete to Castro Urdiales
Ontón is linked by bus with Castro Urdiales (ask locally because I'm not sure if online schedules are updated).
Pendueles has buses to Llanes operated by Alsa:
www.alsa.es/en From Pendueles to Llanes, you have also the option of narrow gauge train operated by RENFE FEVE:
www.renfe.com/EN/viajeros/feve
Alsa has bus service to Ribadesella from Nueva (de Llanes) and Belmonte (close to Cuerres) to Ribadesella.
RENFE FEVE has train service to Ribadesella from Nueva, Villahormes and Belmonte (Belmonte isn't on the Camino but it's very close to Cuerres).
Peón has buses to Gijón operated by
www.autocaresmariano.com Get in touch with them to ask for updated info. If the info on their web was updated, it wouldn't be service on Sundays and public holidays and on Saturdays service would be just one bus (departure in the morning) but check with them the current situation.
Alsa has bus service from Soto del Barco and from Muros de Nalón to Cudillero.
I'm not sure if there's bus service to Santa Leocadia but you could take a bus from Vilalba to Baamonde and walk from Baamonde to Santa Leocadia.