Photo: Sādhaka of Vakreśvara Paṇḍita/Gopāla-guru Gosvāmī Parivāra.
Tilaka-svarūpa of some Parivāras (there are others).
The scriptures do not specify in detail about this.
The material for applying Tilaka in the Gauḍīyā Vaiṣṇava Parivāras is the clay of Rādhā-kuṇḍa.
The Tilaka used is Ūrdhvapuṇḍra.
Ūrdhva means "up" or "upward."
Tilaka/Puṇḍra means "mark".
Here is my personal interpretation of the form of some Tilakas.
In the Advaita Parivāra the Ūrdhvapuṇḍra has at its base a leaf of Vaṭa (Banyan) in the form of heart. The Vaṭa tree represents life. The revered Vaṭa tree is never cut, and thus often grows on many acres. Śrī Advaita Ācārya takes us to the feet of Gaurāṅga (Rādhā Kṛṣṇa) and thus infuses us with true life. This is represented by the Vaṭa (Banyan) leaf. Vaṭa trees are often associated with Śiva. So, the connection with Śrī Advaita Ācārya who is Sadāśiva.
In the Nityānanda Parivāra the Ūrdhvapuṇḍra has at its base a triangular leaf of Nimba (neem). The tree of Nimba has a healing power. Śrī Nityānanda Prabhu takes us to the feet of Gaurāṅga (Rādhā Kṛṣṇa) and thus heal us all from the material life. This is represented by the Nimba leaf. Nitāi is honoring Nimāi who He adores and worships. Lord Caitanya is called Nimāi because He appeared under a Nimba tree to heal us all.
In the Śrīnivāsa Parivāra the Ūrdhvapuṇḍra has in its base a leaf of Tamāla. The tree of Tamāla represents Kṛṣṇa. Rādhā embraces the tree of Tamāla. And thus Rādhā and Kṛṣṇa are united in an embrace. This is represented by the Tamāla leaf.
In Śyāmānanda Parivāra the Ūrdhvapuṇḍra represents the imprint of Rādhā's anklebell with a dot in the middle without leaf.
In Narottama Parivāra the Ūrdhvapuṇḍra has in its base a Tulasī leaf. Tulasī allows us to become maidservants (Dāsī) of the Divine Couple.
In Gadādhara Parivāra the Ūrdhvapuṇḍra has no leaf at its base. The followers of this Parivāra are identified with Rādhā (Gadādhara). Tulasī is not offered at the Rādhā's feet.
In the Vakreśvara Paṇḍita/Gopāla-guru Gosvāmī Parivāra the Ūrdhvapuṇḍra is the lotus feet of Gaura. Also without leaf. Lord Gaurāṅga placed His foot on the head of Gopāla-guru Gosvāmī to protect him.