If they jam GPS over a wide area, they will have a lot of fun with all the aircraft which
- are flying IFR
- are flying in CAS
- are required to be BRNAV compliant
- have an IFR GPS as the only possible/practical means of BRNAV compliance
- have been given a nice 200nm DCT by ATC
That includes everything meeting the above scenario, right up to big business jets. Very very few bizjets have INS or IRS; GPS-based RNAV FMS is the standard fit.
According to a Eurocontrol nav conference I went to a while ago, the planned fallback for loss of GPS is ATC (radar) vectoring, which is fair enough but the ATC workload will go way up if they jam GPS over a wide area and they do it from a reasonable altitude so that airborne aircraft (with proper roof-mounted GPS aerials) are affected too.