Is there a way, using the GUI, to change the default "You can't go there." message when the player is in specific rooms?

for example if the player is in a cave that runs north/south only and they try to go west, it would say something like "The only thing to the west is the solid rock of the cave." but if they are in a forest with no exit to the west it could say "The forest to the west is too dark and scary, you decide against heading that way."

You just need to create an exit, lock it, and set the lock message as the message you want to print.

Using the GUI, I find it's easiest to go to the exits tab for that room, and create a look direction. Then select the newly made exit in the object tree, uncheck the "look direction only" box and then check locked and set your lock message. Making it a look direction first saves you all the faffing about selecting a room for the exit to go to when you'll just be locking it anyway.

Great, worked like a charm. Thanks

And actually I realized that if I leave the exit unlocked, but check the Run a script(instead of moving player automatically) checkbox and have the script print a message, I can have both a look direction description and a trying to move in that direction description.