Gimpo to Haneda

I will be flying from Gimpo to Haneda to connect with a JAL flight to the US.
When booking my flight from Gimpo, how long should I allow to clear customs and possibly collect bags.
Thanks

I recommend that you book at once two flights
which are suggested together as connection by your air company.
https://www.jal.co.jp/krl/en/booking/freedom/

You don't have to enter Japan this time.
https://tokyo-haneda.com/en/flight/transit/int_int.html

Your post is confusing. Clear Customs? So you are staying in Japan? If you are not entering the country, you are only in transit, and don't need to go through Customs and Immigration. If you have separate tickets, you should be able to have your bags checked through to the US if you show both tickets at the ticket counter in Gimpo.

My question probably reflects my own confusion, sorry.
This is the end of our trip. We will not be staying in Japan. Just passing through the airport- in the same terminal.
Trying to decide if 2 1/2 hours is enough to make the connection. Thus my concern about customs and security.
Since we do not leave the terminal perhaps this is not an issue.

Thanks for clarifying. You are only in transit, so you do not go through Customs at all. You simply deplane, go through a security check, and enter the transit lounge. You do not need to collect your bags or go through Japanese Immigration.

Arenft the two flights with the same airline? (If so, then your luggage will be checked through to the US, too.) Then 2 1/2 hours for connection (not entering Japan at all, not collecting your badge either) should be sufficient, though of course there could be unexpected delays.
If you are trying to book the two tickets with JAL, they should show you the recommended connections.

The problem is that the first ticket is already booked on JAL.
Because of a huge difference in ticket pricing I elected to book the tickets separately and now have this quandary re. the Seoul to Haneda flight timing.
If you are certain I wonft have to go through immigration and customs upon deplaning I will book the later ticket.
2 1/2 hours should be ample time, donft you think?

But I do think I would have to collect my bag as I am on two separate JAL ticketsc