Sending money overseas thru GCASH is not yet possible. GCASH allows sending and receiving to Globe subscribers which is domestic carrier.
As long as the booking site caters MASTERCARD, pwede.
Hi el_ri0, if the person you're sending money to is a Globe customer and he has a GCash account, you can transfer money to him through *143# or the GCash mobile app.
As for flight booking, you can register to GCash AMEX and use that for flight booking, since you will be given a virtual credit card number (as if you have a real credit card).