Gravity Fields
Unfortunately, linked_portal_door entities can only take up a maximum grid of 2048x2048. You should really only one linked pair at a time if you want to use portals between the 2 areas (due to portal limits).
If your game is crashing, then one or more of those WP's aren't linked. You should be able to manage 2 WP's in the same room without too big an issue.
Normal Gravity............Altered Gravity
|-------ceiling---------| |-------ceiling---------|
|.......................| |.......................|
|.......................| |.......................|
|.......................| |.......................|
|.......gravity v.......| |.......gravity ^.......|
|.......................| |.......................|
|.......................| |.......................|
|.......................| |.......................|
|____floor______| |____floor______| (ignore the dots)
This will be a triggerable thing. I just don't know how to accomplish this.