Fentanyl is an opioid, albeit a synthetic one. The CDC numbers don't break down the percentages of which drug killed the most people. The California site that you cited does, but the 5,502 number you cite is "Deaths Related to Any Opioid Overdose", which includes Fentanyl. So according to the link you cited, nearly 72% of all opioid deaths (in California) were caused by Fentanyl. The California firearm death count you cite is still lower than California's Fentanyl death rate for the same period. Extrapolating for the whole country using California's Fentanyl/Opioid ratio yields an approximate US Fentanyl death count of 65,838 for 2020.

https://www.dea.gov/factsheets/fentanyl

"Rates of overdose deaths involving synthetic opioids other than methadone, which includes fentanyl and fentanyl analogs, increased over 56% from 2019 to 2020. The number of overdose deaths involving synthetic opioids in 2020 was more than 18 times the number in 2013. More than 56,000 people died from overdoses involving synthetic opioids in 2020. The latest provisional drug overdose death counts through June 2021 suggest an acceleration of overdose deaths during the COVID-19 pandemic."

https://www.cdc.gov/opioids/basics/fentanyl.html