Because Canaries and pet finches remove the husks from their seeds before swallowing, there is very little indigestible material in their diet. For this reason they do not need insoluble grit to help them grind up and digest their food - even though some websites still recommend adding it to your birds' cages.
Neither Gouldian finches nor any other species of pet finch need soluble grit
Like all birds, a finch’s stomach has two compartments, including a section called the gizzard. This is the bit that grinds up food. Many standard budgie seed mixes include small bits of soluble grit, usually oyster shell. The bits of shell are dissolved in the gizzard over time and act as a calcium source for the bird. Your bird’s calcium needs, however, should be covered via a cuttlefish bone and a mineral-fortified seed mix, so even soluble grit isn’t essential.
If a bird eats too much grit it may die of a blocked crop. The best policy is to never offer insoluble grit, as it serves no purpose and may even lead to problems. If you give them soluble stuff, such as the oyster shells mentioned above, make sure it’s in very small pieces. Check the makeup of your mineral block, though, and if this provides sufficient calcium, don’t bother with grit of any kind.