From which temperature and above I have to perform stress analysis for GRP piping?

Ambient temperature - 21 deg C

Please guide us.

That's up to the end user, and would default to their judgement if there are no written specifications. It is not uncommon to analyze all GRP for some clients, for example.

For glass lines pipes we considered all the pipes as critical.

Just for information.

Regards,
R.K.