I installed VisualGDB 5.2 Preview 1, and it appears that some of the build variable names changed. For example, a project that was created with an older version of VisualGDB has a build variable named $(ARM_TOOLCHAIN_ROOT). It appears that was changed to $(TOOLCHAIN_ROOT) in 5.2 Preview 1.
Did the tool chain root variable get changed in 5.2? I might be dealing with a separate problem, but this seems the most likely.
The $(TOOLCHAIN_ROOT) is a new variable that is now automatically set to your toolchain location. You can still use the old ARM_TOOLCHAIN_ROOT variable if you want to use the toolchain relocation mechanism, but VisualGDB will automatically substitute $(TOOLCHAIN_ROOT) in directories reported by the toolchain.