Love and mutual agreement are the only necessities. It is not at all necessary to have sexual intercourse prior to marriage, which is of course Adharmic, where as the Gandharva form of marriage is one of the allowed forms of marriage.
इच्छयाऽन्योन्यसंयोगः कन्यायाश्च वरस्य च ।
गान्धर्वः स तु विज्ञेयो
मैथुन्यः कामसम्भवः ॥ ३२ ॥
icchayā'nyonyasaṃyogaḥ kanyāyāśca varasya ca |
gāndharvaḥ sa tu
vijñeyo maithunyaḥ kāmasambhavaḥ || 32 ||
The mutual union of the bride and bridegroom, through love is to be
known as the “Gāndharva” form; it has sexual intercourse for its end
and it has its source in lust.—(32).
Manu Smriti 3.32
It says the goal of this marriage is sexual intercourse and the motive arises from lust.
An alternate translation of this verse is as follows:
3.32. The voluntary union of a maiden and her lover one must know (to be) the Gandharva rite, which springs from desire and has sexual
intercourse for its purpose.
So, sexual intercourse is the purpose not a requirement.
Various verses about the same can be found in the link given above and are as follows:
Gautama (4. 10).—‘When the girl loves a man and herself becomes
united to him, it is the Gāndharva form.’
Baudhāyana (1. 11. 6).—‘The Gāndharva consists in the mutual union of
the loving bride and the loving bridegroom.’
Āpāstamba-Dharmasūtra (2. 11. 20).—‘When the couple become united
through mutual love, it is the Gāndharva.’
Vaśiṣṭha (1. 33).—‘It is the Gāndharva when the man loving the girl
who loves himself, and is similar to himself, marries her.’
Viṣṇu (24. 23).—‘When ṭhe couple in love with one another, become
united, independently of the parents,—it is the Gāndharva.’
Yājñavalkya (1. 61).—‘The Gāndharva is accomplished by mutual
agreement.’
Āśvalāyana-Gṛhyasūtra (1. 6. 1. 5).—‘It is Gāndharva when the man
marries the girl after coming to a mutual agreement.’
Devala (Vīramitrodaya-Saṃskāra, p. 855).—‘When in a sacred place, the
man and the woman become united by mutual agreement, through love, it
is the fifth form of marriage, the Gāndharva.’
Hārīta (Do., p. 856).—‘When the girl herself selects the bridegroom,
it is Gāndharva.’
