hmm, let me attempt to help out with ur questions. but i'm sure our board moderators will be able to help u more later on.
1. Which RO model are more close to GG original design for AP, with chrono or w/o chrono??
- the closest is the RO jumbo 15202st. the original GG design for AP has no chrono, just a clean dial with date (doesn't even have seconds hand if i'm not mistaken). unfortunately it's pretty rare and selling at premium if i'm not mistaken. should be about 12.5k USD for LNIB/BNIB. but there's an cheaper alternative which is the 15300ST, it's the newer model with the new inhouse calibre 3120 (which is currently being used in the new AP ROO diver and slowly phasing in for the other ROO models). beautiful movement with 22k gold rotor and display case back. this one may be in ur price range as i've seen BNIB for 8k USD or so.
2. GG design both VC Oversea date and Oversea Chronograph?
Genta only designed the original overseas model (not chrono). this is the youngest GG design amongst the big 3.
3. Ingénuer 3227-01 is the only one design by GG for IWC??
i'll leave this one for chrisyen to help. i'm not familiar with the ingeniuer range.
4. Nautilus was design by GG and how about Aquanut?
the aquanaut was not designed by gg, i read that it's a simpler version (base model for pateks). the aquanaut is really stripped down version of all pateks, with no complications & only a rubber "tropical" strap. but still very expensive because of the patek brand, i think is out of your budget if you're looking for aquanaut jumbo (which is closest to the nautilus design).
5. Which GG design watches would you go for it with a budget of under RM 30k(for both brand new and preown)?.
it is still down to you and which sings to you. i personally didn't really like to ingeniuer series, but love the royal oak range. problem is if you have a small wrist, it doesnt fit that well because of the way its lugs are protuding out of its case.
check out the royal oak 15300st, i think the MSRP locally is 45k (with decent discounts). slightly above ur budget, but u can search for one overseas, roughly 8k USD if i'm not mistaken for A BNIB.