No, not all G20 summits are attended by 100% of its members. In some cases, leaders may choose not to attend due to scheduling conflicts or other reasons. For example, in 2014, Russian President Vladimir Putin did not attend the G20 summit in Australia due to tensions between the two countries over the annexation of Crimea. In 2022, Russian President Vladimir Putin did not attend the G20 summit in Indonesia due to the ongoing war in Ukraine.
In addition, some countries may be represented by a lower-level official, such as a finance minister or a foreign minister, instead of the head of state. This is often the case for countries that are not major economies or that are not members of the United Nations Security Council. For example, in 2019, the leaders of Argentina, Brazil, and South Africa were all represented by their finance ministers at the G20 summit in Osaka, Japan.
Overall, attendance at G20 summits is not mandatory and is ultimately up to the discretion of each member country.