Are eggs from gift predetermined before you open the gift?

No.

I received gifts from 5 friends september 10th 2018 (before de event). I didn't accept gifts or received eggs from Pokestops for several days.

The september 13th 2018 opened the 5 gifts getting 5 eggs of 7 km. After hatching them they were:

• 2 Alolan Diglett
• 1 Alolan Sandshrew
• 1 Mr. Mime
• 1 Tangela

The Egg is determined when you receive it. Many people have received regional Pokemon that were only available during a specific time window from opening gifts within the event time, even though those gifts were received before the event time.