Is there a function to get the max of two values in Google BigQuery?

There isn't currently a function to return the greater of two values in BigQuery. If you end up needing to compute the value a lot in a single query, you can always get the greater value in a subselect.

For example:

SELECT gr 
FROM (
    SELECT IF(column1 > column2, column1, column2) as gr 
    FROM [my_dataset.my_table])
WHERE gr > 27
GROUP BY gr

BigQuery supports

GREATEST(expr1, expr2, ...) 

which returns the largest argument. I've filed an internal bug to get this added to our public documentation.