inform ATC about TCAS/GPWS failure?
 

is there any legal requirement about informing ATC in case TCAS or GPWS fails?

No. Even if the gear remains down, not mandatory to report to ATC;)

TCAS: Try MEL 34-46-01. Our Civil Aviation Regulations state the requirement to advise ATC of a failure in flight, and before departure.

GPWS: No requirement here.

You sure about that TCAS is

Depending on where in the world maybe don't. Informed ATC once and they insisted we descend out of RVSM! After much explaining, to no acceptance, it miraculously started working again so we remained at our level.

Inop TCAS is normally a 10 days MEL item. Most countries in Europe have no problem operating for 10 days with u/s TCAS except Germany which have a quite stupid rule of max 3 days (altough even their ATC guys are not aware of this apparently). They have no way to find it out either unless your company run at least a half-decent Operations Control who files your flight plans marking TCAS being inop (non-RVSM).

It used to be france, germany and some other country (uk?) that allowed max 3 days with TCAS inop. ATC might not have a way to check it, but SAFA inspectors do and they will not only record a finding, they will ground the aircraft until a repair is done if it is over three days.

It's 10 days in the UK as well given that the transponder is operational.