By the way, how hard is it to jam a LOC or GP? Really do not know, but would think they send with quite a bit of power.
A key difference between an ILS or VOR and GPS is that the former two are local radio navigation aids, whereas the latter is worldwide. To jam an ILS or VOR, then, you need local jamming transmitters, one for each VOR or ILS. To jam GPS, you just need a single powerful transmitter for a large area. The GPS signal is many times weaker than the ILS or VOR signal, which also helps to make it far easier to jam or spoof. GPS also uses a very limited range of frequencies and produces a signal that can only be distinguished from background noise using special detection techniques, both of which make it more vulnerable to interference.
Jamming the signal removes the ability to use GPS, which is quite a hazard in itself if you are depending on GPS (no ILS or VOR available, which is already the case where they never existed, and might be the case in the future if the authorities continue to recklessly decommission stations). Integrity monitoring may be able to detect a jammed signal, but it cannot replace it if the jamming is well done, so even if it detects the loss of the signal, you still have no GPS.
Spoofing is almost as easy and much more sinister. With spoofed GPS, it seems that everything is working and integrity monitoring may not notice anything strange, but you are no longer where your GPS says you are, and obviously on an approach in particular, this can be deadly. Spoofing GPS itself and WAAS augmentation is straightforward; spoofing LAAS is more difficult because of the local transmitters (as with ILS and VORs).
The cure to spoofing is encryption, and the military has used this for years, but it is not practical to implement it for civilian aviation.
For all of these reasons, having GPS as the sole means of navigation is very unwise. For the same reasons, decommissioning LORAN was a bad idea, as is any decommissioning of VORs or ILS.