In plain words
Among Sama hymns I am the Brihat-Sama; among metres I am Gāyatrī; among months I am Margashirsha; among seasons I am the flowering spring.
Word for word
The Bṛhat Sāman likewise among sāmans, the Gāyatrī among metres [am] I; among months I [am] Mārgaśīrṣa, among seasons the flowering [spring].
The commentators
Bhagavadgītābhāṣyam· bhāṣya
बृहत्साम तथा साम्नां प्रधानमस्मि । गायत्री च्छन्दसाम् अहं गायत्र्यादिच्छन्दोविशिष्टानामृचां गायत्री ऋक् अहम् अस्मि इत्यर्थः । मासानां मार्गशीर्षः अहम् , ऋतूनां कुसुमाकरः वसन्तः ॥ ३५ ॥
In plain words
Among the sāman chants, I am the Bṛhat-sāman, its very foremost. Among poetic meters, I am the Gāyatrī — that is, among verses composed in meters beginning with Gāyatrī, I am the Gāyatrī verse itself. Among months, I am Mārgaśīrṣa. And among the seasons, I am spring, the very maker of flowers.
Close to the Sanskrit
Bṛhat-sāman, likewise, among sāman chants, I am the foremost. Gāyatrī, among meters, I — of verses distinguished by meters beginning with Gāyatrī — Gāyatrī, the verse, I am; this is the meaning. Among months, Mārgaśīrṣa, I; among seasons, the producer of flowers, spring.