Hey there - I've been playing around with a "seamless world" myself. Its far more complex that I ever realized it would be.
As many of the people above have mentioned, you really have to break the world down into smaller pieces first - all of the objects in the entire world (depending on the size of your game) would likely be far too much for both GM and the average computer to handle. So, creating a system to dynamically save/load your content would be the way to go.
Here is the approach I am attempting to program currently -
Note - I'm in the very early stages of development on this, so it may not work... But it looks good on paper to me ..
Lets say your "overworld" is one large map that could be equally divided into 16 smaller pieces.
These smaller pieces are individual rooms in GM.
Something roughly like this =
[01] [02] [03] [04]
[05] [06] [07] [08]
[09] [10] [11] [12]
[13] [14] [15] [16]
Lets say the player is in room 11.
You'd want to have rooms 10, 7, 12, and 15 loaded as well.
That way the player can move in any direction from his current position, and the content would already be ready to go.
So, lets say the player moves to the room to the left of room 11...
Room 10 is already loaded in memory, allowing you to move to that location.
From there, you would drop room 12, and have rooms 9, 6, 11, and 14 loaded.
So basically, you are always dynamically keeping all of the rooms surrounding the room you are in loaded and ready to go.
I believe this is a really rough overview to how AAA titles get away with these massive open world games that are so popular these days.
They are only loading content that is within x radius of the player - no sense in wasting resources on items that are on the opposite side of the world from the player.
Just store that information, and load it when you need it.
And keep in mind, that even though GM isn't the most limitless game engine - it can do quite a lot.
The rooms in the prototype I am currently working on are all 10,000 x 10,000 px in size - which is quite large..
Now if you take that room and add it to 63 other rooms of the exact same size - you've end up with one massive map at 80,000 x 80,000 px. That should be a good starting point for your RPG world..
Good luck!