I have finally found my note from that flight to Sardinia in 2010.
What harmonic of 135.675MHz could affect the GPS frequency of 1575MHz?
Presumably it is something to do with the DME frequency which is auto-tuned when one selects 135.675 on the NAV radio.
I can't find a pairing for 135.675. The closest pairing I found 13Y 135.65 is paired with 1037 MHz and 14X 135.7 is paired with 1038 MHz. The range of the DME transmit frequencies was 1025 MHz on the low end and 1150 MHz on the high end.
That's really weird.
The frequency does not appear to be allocated to any navaid in Europe, currently.
The radio was a KX155A and the DME connected to it was a KN63. I suppose one could investigate via Honeywell but I am not aware there are any engineering resources in there (Olathe) anymore. I have the MM for the 155A but AFAIK that just outputs the knob setting to the DME. I don't know about the KN63 but the KN75 uses some PALs to implement the lookup tables for the DPLL tuning. It's also possible that one can set a VHF frequency which does not produce any pairing, and the DME does something slightly weird.
Also note that the frequency is in the VHF Com band and not in the VOR Nav/Localizer band of 108 to 118 MHz range.
Most GPS interference caused by VHF Comm frequencies is attributable to the direct signal not harmonics which would be so weak that they would be insignificant. Most GPS antennae are active and any component combination that might lead to a resonance in the VHF range could cause the active antenna to be switched off by saturation. Just put a VHF hand held near a GPS antenna and you can demonstrate this.