A custom good is a product that you have made yourself, in other words, not mass produced. This can be a painting, crafts, clothing or any other hand made product.
Products that are made to order are also considered custom goods.
In this situation no, along side GTIN you should also not fill in the brand and mpn. But you do require to submit identifier_exists to false. The quickest way to check if you need to add the value GTIN to Google Shopping is by checking the packaging of the product. Does it have a bar code? If the answer is yes, than you need to fill in all product identifiers else set identifier exists to false.
An easier way of listing the required products is to list the ones that do not require one.
With apparel goods you are required a GTIN on products inside the following categories
All other products not mentioned above require at least two of the three required fields (Brand, GTIN or MPN).