What if GAAP and IFRS reporting are used in subsidaries at different locations, what reporting standard will the head office follow in consolidation?

It depends on the country which  the head office located

dear respectables,

as an example i will quote financial statement of BMW there they have presented segment reporting where part of the segment was dealt with ifrs and at HO with german local accountancy law, the adjustments in these scenario may fluctuate their curreny and profitablilty differences.

do correct or answer? 

It is the job of the Head office to instruct all its branches and subsidaries to follow a Standard method of accounting process to ensure they all use common modules which would make consolidation easy for head office.

In the above case; the head office is to identify the gaps in the accounting practise and pass the necessary adjustments to reconcile the balance sheet and P&L for accurate consolidation.