Something is safe or unsafe is a matter of time till its not broken but as of now we can consider it as safe. GbWhatsapp is not just another messenger application but it is a modded version of original whatsapp.
In Android APK, Stylesheet information is NOT ENCRYPTED but the codes are encrypted. In GbWhatsapp you will see some customization and some more features but to send and receive message GbWhatsapp use original whaqtsapp server. So you are safe while using GbWhatsapp.