I guess, but Dr. Robotnik was the former leader of G.U.N., someone would';ve informed him.
If not Stone, he would've likely found out.
He's probably been informed but didn't really care.
He started to care about this "family" thing after watching a lot of latina telenovela in his free time.
Eggman's only surprised when meeting him because Gerald's still alive and fresh since nobody (Eggman included) thought someone who was born in 1914 would be still alive in 2024.

The director, Jeff Fowler, revealed in an interview that Gerald Robotnik's longevity was assisted by Shadow's quill.
He is regularly shown licking one of Shadow the Hedgehog's quills that he has held onto for 50 years, after the tragic death of Maria.

Little Japanese girls will stop Pikachu in the streets and ask if he's Tails.
(Detective) Pikachu probably won't be annoyed by that though, since there's no Knuckles-like figure who dares to laugh about it besides him.