Kindly please educate me on this reptile which was said in fiqhihi (zubdatul Ahkaam) of Ayatullah Imam khumaini(r.a) that ghusul janabah is compulsory upon you when you kill it and it’s tail was cut off?

Ghusl e Janabat can’t be taken after killing any animal particularly a reptile.
It is fact that whenever any Marja e Taqlid wants to give any fatwa as a wajib then he must have seen an authentic Rewayat (tradition from Holy Prophet or Aimmah a.s) otherwise just for Mustahab act they follow any rewayat. As for as Killing reptile ( Marmoolak in Farsi or Soosmar) the rewayat has come are weak in their chain of narrations and if Ghusl has been recommended after killing them is just because of cleanliness that you may consider as a simple bath but there is no Ghusl e janabat has been recommended after killing a reptile.
There are some traditions has come regarding reptile that they are the animals who got “Muskh” therefore after killing them you may do Ghusl but these rewayat are not much authentic and we have tradition from Imam Sadiq (a.s) that without any genuine reason none can kill any animal or insect.