At GG cup a nursing bra has to solve two problems at once: support a fuller bust properly, and open one-handed while you are holding a baby. Plenty of nursing bras manage the second and not the first.
What a GG cup nursing bra needs
Look for a structured, sectioned cup rather than a single stretch panel, a firm band that takes the weight off the shoulders, and clips you can work without looking. Because the bust changes size through the day, a cup with a little give — flexi-wire or a well-cut wirefree — is often more comfortable than a rigid underwire.
Brands and styles
Hotmilk and Cake Maternity range GG properly. See wirefree maternity bras options and the wider maternity and nursing range.