"IDN is the short name for an Internationalized Domain Name. These domain names are represented by local language characters. Such domain names could contain characters with diacritical marks as required by many European languages, or characters from non-Latin scripts (for example, Arabic or Chinese)." *
In the case of ccTLD .gt domain names, the following letters in the Spanish language are available for the domain names :
á, é, í, ó, ú,
To register an IDN write it as it is in Spanish while searching for its availability .
* For more information see factsheet-idn-program-05jun09.pdf