What is a 36G bra equivalent to?

A 36G bra is similar in size to a 38DDD or a 34H. These are known as sister sizes. A sister size is when you go up in band size and down in cup size, or up in cup size and down in band size. 

When you choose a sister size, you'll have the same cup capacity, but the band will either be tighter or looser. In this example, a 38 band will fit more loosely, and a 34 band will be tighter, but the cup volume will remain the same.