Relevanssi doesn’t work automatically with the Query Loop block in GeneratePress and GenerateBlocks. Fortunately, the solution is simple: the Query Loop block has a filter hook for adjusting the query parameters, and you can use that filter hook to enable Relevanssi.
add_filter( 'generateblocks_query_loop_args', function( $query_args, $attributes ) { if (! is_admin() && ! empty( $query_args['s'] ) ) { $query_args['relevanssi'] = true; } return $query_args; }, 10, 2 );
Thanks to Ben Knowles.