Is it valid to perform Shradh in Gaya for ancestors whose tithi (death date) is not known?

Yes. When the exact death anniversary tithi is unknown — a situation common for many NRI families — the ritual can be performed on Amavasya (new moon day) or during Pitrupaksha, both of which are universally auspicious for all ancestors regardless of individual tithis. The Mahalaya Amavasya, which falls on the last day of Pitrupaksha, is specifically designated for ancestors whose tithi is unknown. This makes it the perfect day for Malaysian families in this situation.