Here's what I would do if I knew which walls were slide-able.
+On your trigger
->wallWithGap pick closest to player.x,player.y
-->wallWithGap compare x wallWithGap.x >player.x+offset
--->player compare angle player.angle is 0: do stuff
-->wallWithGap compare x wallWithGap.x <player.x-offset
--->player compare angle player.angle is 180: do stuff
Edit:
Now that doesn't take y into account, so if you have multiple levels of walls, IE walls that can be slid under above and below the character, you will need to compare the y as well.