Logging into your GLPI installation from Gapp (A GLPI app) is not possible due to this error message:
missing parameter app_token; view documentation in your browser at http://yourglpi/apirest.php/#ERROR_APP_TOKEN_PARAMETERS_MISSING
As for GLPI API Errors reference:
ERROR_APP_TOKEN_PARAMETERS_MISSING The current API requires an App-Token header for using its methods.
An App_Token has been generated for a particular API Client, and it has not been implemented in Gapp.
Removing Application token (App_Token) will fix the error.
Don’t forget to save!