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(1) Do I require the transit visa and from where I would get it (Gimpo Airport or Korean Embassy in Osaka, Japan)?
Depending on your citizenship, if you are not visa free to Korea you will need a transit visa as your layover is longer than 6 hours and you will be switching airports.
You would apply at the Korean embassy in Japan.
(2) How would I transfer from Gimpo to Incheon airport, what would be the fare and how much time it takes?
See this guide here:
You can take the train from Incheon to Gimpo.
The line is named AREX
Upvote:1
In answer to the second of your questions, there is a train between the two airports that takes approximately 35-40 minutes. It costs approximately 3500 Won.
I've had a look regarding your first question and I can find no evidence that visas are available on arrival in Gimpo. As an Indian national you would require a visa to enter Korea (which you must do to change airports). Therefore, you would need to apply for a transit or tourist visa from the nearest Korean Embassy or Consulate to your residence.