Should guest relation officer know the details of work of front office? Should she handle wourk of receptionist?

Yes! should be the front receptionist aware every details or every department, just in case they may ask him/her any information she can give the exact answer with hesitation

She/ he must be aware of other job positions. also, have a training about it and be aware that it is not supposed to be said " It is not my job, " or " can You , please wait until my colllegues returns from brake". He/She must be smart to find a soulution in any case, to help the other departaments. it is not so different kind of job, either it is not that she/he would be asked to be a accountant instead of one. It is all about team work, proper education in all this fields and departament.

Broader roles are becoming frequent trends in companies due to cost cutting measures and interconnecting roles. It's certainly is more efficient and productive to have GRO having to do limited reception work on occasion than it is have your guest/s waiting in the lobby.