capital gain on sale of foreign securities

First you examine the DTAA w.r.t. Capital Gains:-

USA
Article 13 - Gains - Except as provided in Article 8 (Shipping and Air Transport) of this Convention, each Contracting State may tax capital gains in accordance with the provisions of its domestic law.

UK
ARTICLE 14 - Capital Gains - 1. Except as provided in Article 8 (Air Transport) and 9 (Shipping) of this Convention, each Contracting State may tax capital gains in accordance with the provisions of its domestic law.

Most of the contracting states including India tax the capital gains arising on transfer of the properties in their jurisdiction. You will have to check the USA laws from IRS website or UK Laws from their tax website and see the situation of taxing resident's / non-resident's capital gains there. You have to pay the tax there if you have to.

As India taxes its Residents on Worldwide income, the Capital gains will be taxed here and you will be given the relief of the tax paid in the foreign country to the extent of Indian Tax on the same income that suffered tax.

can you please mention how the capital gain will be computed under income tax in India?

like :

1.  what will be the holding period to compute short term or long term?

2. If LTCG,Is indexation available on such gains?

Example :

Suppose Mr.X (resident)  buys Foreign Currency Money market funds maybe from kuwait  which he intends to sell after  3 years.

now after 3 years this will be LTCG

is indexation available on such ltcg?

please provide section in support of your answer??