This is a specific question. Please do not quote Smriti Shastras. I want to know what Krishna said about inter-caste marriage in Bhagavad Gita. Was he in favour of it? Or did he criticize it?
Sri Krishna did not address inter-caste marriage in the Bhagavad Gita; that is not the purport of the Bhagavad Gita. Sri Krishna does address marriage in the Uddhava Gita Chapter XII (Swami Madhavananda translator). In this chapter, He first addresses what the qualities, or tendencies, of each of the castes. He defines caste as a person's tendencies, not their birth. After first addressing this, He then talks of marriage in verse 39. Sri Krishna says:
A person wishing to lead a householder's life should marry an unblemished girl of the same caste, who must be younger in age; and if he wishes to marry other, he should do so after the above marriage, and even then, in succeeding order. (Succeeding order - i.e. a Brahmana was allowed to marry in the three lower castes also, a Kshatriya in the two lower, and a Vaishya in the Shudra caste also, the last being confined to his own caste. But not in the inverse order.)
Shri Krishn opposed inter-caste marriage in the context of varnashram-dharm. In the context of Bhagavat-dharm, He neither criticized nor supported it. Furthermore, if we extrapolate marriage to the realm of Jeeva-Bhagavan-relationship, He was all for it.
Jeeva has no gender, or rather, is feminine on account of being Shakti of Bhagavan. As such, all jeevas are eternally related to Bhagavan in the conjugal mood. Thus, to the external view, Krishn undertook inter-caste marriages Himself, but as pointed above, they were perfectly in line with varnashram-dharm. In other words, He didn't marry a Brahman girl. However, even that would be perfectly alright if it had happened. Because jeev is jeev, whether she is inhabiting the body of a Brahman girl or a chandal girl or Kubja or even men. The maha-raas is transcendental to the Varna-ashram dharm and thus cannot be defined in terms of 'inter-caste' transactions.
In the context of jeev-jeev relationships, inter-caste marriages are a strict no-no, unless a brahman girl finds a shoodra husband who is a sincere and unflinching devotee of the Lord (aka Vaishnav). In such a case, the Brahman girl can marry anyone, so long as his mind is completely and utterly fixed on Lord Krishn/Raam/Vishnu. Pati-vratas obtained svarga due to their intense attachment to their husbands, who were sattvik in nature. A Brahman bride would obtain the nirgun-phala that her chaandal Vaishnav husband would attain, on account of his bhakti, and not hell (or naraka) which such a marriage would entail under normal circumstances. This would happen only if the Brahman girl was attached to her Vaishnav husband, and not otherwise. The jeev attains that phala to which she is attached on earth.