Your research results look a bit odd to me. Lower heat range, not higher, ie one step colder. Different plug manufacturers use different number ranges that go in different ways. In Champion you'd want a 9, NGK a 6 and Bosch a 7. "Brisk" make a LPG specific plug that seems to be good, I've fitted a couple of sets of DR15YS and they're running well. Available here.
Gaps should be around 0.7mm to 0.8mm for LPG. And stock standard OEM style plug leads are best, Roverlord sells them. Racing type leads have too little electrical damping and resonances can cause all sorts of grief.
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