Hi there,
the Customizer > Typography H2 sizes will apply to the standard H2 heading.
For example when adding a Heading Block to your page.
But if you use a GenerateBlocks Headline Block, it has its own block editor controls that you need to set the font size for.
Alternatively swap the Headline block for a core Heading Block.
NOTE: We recommend only using the GB Headline block instead of the core paragraph / heading blocks where you need to break from the theme styles or do extra fancy styling.