KRAS gene and K-Ras Mutations
NRAS is indeed a molecular switch, as part of the MAP-Kinase signaltransduction pathway it acts in controling the signal which goes downstream and finally will cause the expression of genes. In the case of NRAS this includes genes for proliferation, which is important for tumors. In principle, this looks like shown on this figure (from here):



RAS (this is the same for all three RAS family members) needs to be activated so it can signal downstream, this happens, when GTP is bound to the protein. Since permanent activation is obviously not a good idea, RAS itself has an intrinsic, but slow GTPase activity, which is activated by GTPase-activating proteins (GAP) and will eventually cleave the terminal phosphate group group from the GTP to make GDP + P. GDP bound RAS is inactive. GDP stays bound to the RAS protein and needs to be exchanged against GTP by Nucleotide Exchange factors (NEF) to activate RAS again.

The general principle is shown in this figure (from here):



Mutations in RAS proteins occur in 98% of the time in three locations (see reference 1): In codons 12, 13 and 61. All three mutation affect the ability of the protein to break down GTP by abolishing the intrinsic GTPase activity by the GAP (see reference 2). This leads to the accumulation of RAS protein which is permanently bound to GTP and therefore activated, subsequently activation transcription of genes for proliferation (amongst others). This process can lead to the formation of cancer.

Your questions: 


The conversation of GTP to GDP does not lead to cancer. This process stops the activation of RAS proteins. It is the permanent activation which causes the problems.
You have the mutation in the KRAS gene, which is the origin of the problem. The wrong activation is of course done by the KRAS protein.
GEFs facilitate the exchange of GDP to GTP, the break down of GTP is supported by the GAPs. The mutation hinders the GAP to fulfill their role, so they are useless in the mutated form of the protein.
The other way round. If RAS proteins are bound to GDP, they are rendered inactive, unless a GEF exchanges it again to GTP.


References:


RAS isoforms and mutations in cancer at a glance
Guanosine triphosphatase activating protein (GAP) interacts with the
p21 ras effector binding domain.