It's okay to use a white-out to correct mistakes when writing Gurbani. In olden days, they used to use a material similar to white-out and it was called Hadtaal.
But remember that whiteout is only good for small and minor mistakes. If you write a whole word incorrectly, it's better to use labels. Labels are White labels (that stick) that are used for writing addresses on letters. Address is typed on a label and then it is sticked to the letter. I cut the label to the size required, with scissors, and then paste it over the incorrectly written word. Then write over it. It blends in perfectly. Even the printed saroops have corrections done with labels.
In case you make bigger mistake like skipping a whole line, then it's better to write the whole Ang over again.
Kulbir Singh