As outlined in the New gTLD Applicant Guidebook (Section 22.214.171.124), the GAC can use Early Warnings to send a notice that an application may be regarded as potentially sensitive or problematic to certain governments or that the application could violate national laws. These Warnings are only notices and not formal objections, and as ICANN stated when it filed the Warnings, the Early Warnings "mainly consist of requests for information, or requests for clarity on certain aspects of an application."
Applicants that receive Early Warnings will not necessarily have to change their applications. However, these applicants should communicate with the country that filed the Warning to determine how to work through the issues raised. The GAC page features links to each of the individual Early Warnings, that explain the reasoning behind the Warning.
Again, it is important to note that these Warnings are not formal objections. Rather, receiving a Warning increases the likelihood of being the subject of GAC Advice at ICANN's next Public Meeting, in Beijing, China, in April 2013. Similarly, even if a company did not receive an Early Warning, it may still be the subject of GAC Advice in April.