Updated - March 22, 2021
Tourists that have been fully vaccinated will simply need to present their COVID-19 vaccination card showing that they have had all the necessary doses and they will be allowed to enter Ecuador without any further restrictions or tests. Travelers who are not yet vaccinated will have the option of showing the negative result of an antigen test or a negative PCR test taken no more than 72 hours before arrival.
For now, this change of entry requirements is only for mainland Ecuador meaning that you will still need a negative PCR test taken no longer than 4 days (96 hours) before traveling to the Galapagos. However, it is expected that Galapagos to follow the same or a similar approach as the mainland in the upcoming weeks.
The CDC is still requiring all passengers who board international flights to the US to show a negative PCR test within 3 days of boarding the flight. Therefore people traveling to Galapagos need to add an additional day on the back side of the tour to allow for time to get their test results back from the labs in time for their flight home. I expect that this requirement will change soon for vaccinated people.