Yes, break will work. However, you may find that many programmers prefer not to use it when possible, rather, use a conditional if statement to perform anything else in the loop (thus, not performing it and exiting the loop cleanly)
Something like this will achieve what you're looking for, without having to use a break.
while(choice!=99) {
cin >> choice;
if (choice != 99) {
cin>>gNum;
}
}