Does the said law (or any other law or treaty) prohibit Indians to get the pre-natal gender screening test done outside India (in any country where this is legal)?
YES, in theory but I cannot find any relevant case law where this has been considered by the court.
Section 23(3) of the Pre-Conception and Pre-natal Diagnostic Techniques (Prohibition of Sex Selection) Act 1994 (PCPNDT) creates the offence for non-medical practitioners etc:
Any person who seeks the aid of any [medical practioner etc] or any other person for sex selection or for conducting pre-natal diagnostic techniques on any pregnant women for the purposes other than those specified in sub-section (2) of section 4, he shall, be punishable with imprisonment for a term which may extend to three years and with fine which may extend to fifty thousand rupees for the first offence and for any subsequent offence with imprisonment which may extend to five years and with fine which may extend to one lakh rupees.
And section 4(1) of the Indian Penal Code (IPC) provides for extra-territorial jurisdiction for any offences committed by:
any citizen of India in any place without and beyond India...
Normally, criminal justice action would only be considered once the parties returned to India, but note that section 299 of the Code of Criminal Procedure 1973 allows for trials in absentia. However I cannot find any relevant case law to say whether this has actually happened in this type of scenario.