Appointed as principal of Judith Lady Montefiore College, Ramsgate, from 1891 to 1896, he wrote valuable collection of essays accompanying the yearly reports of that institution.

[3] In 1925, Gaster was appointed one of the six members of the honorary board of trustees (Curatorium) of the Yiddish Scientific Institute (YIVO) in Vilnius alongside Simon Dubnow, Albert Einstein, Sigmund Freud, Edward Sapir and Chaim Zhitlowsky.

He visited Nablus in the Ottoman Beirut Vilayet, the headquarters of the Samaritan community, and induced them to part with manuscripts covering the whole range of their literature.

Gaster was among the most active leaders of the Zionist movement in England, and even while in Romania he assisted in establishing the first Jewish town in Israel, Zichron Ya'akov.

Gaster's residence, "Mizpah" 193 Maida Vale in London served as the venue for early talks between prominent Zionists and the Foreign Office in 1917.

The first draft of the Balfour Declaration was written at the Gaster home on 7 February 1917 in the presence of Chaim Weizmann, Nahum Sokolow, Baron Rothschild, Sir Mark Sykes and Herbert Samuel.

[4] The collection comprised over 10,000 fragments in Hebrew and Judaeo-Arabic from the Cairo Geniza (the genizah of the Ben Ezra Synagogue in Old Cairo); some 350 Hebrew codices and scrolls including prayer-books of many Jewish communities, apocryphal writings, commentaries, treatises, letters, marriage contracts, piyyutim, and thirteen scrolls of the Law; some 350 Samaritan manuscripts, among them manuscripts of the Pentateuch, commentaries and treatises, and liturgical, historical, chronological and astronomical codices, detailed census lists of the Samaritans and lists of manuscripts in their possession; and almost 1,500 uncatalogued Arabic fragments on paper from the Synagogue of Ben Ezra.

Gaster also wrote various text-books for the Jewish community of Romania, made a Romanian translation of the Siddur, and compiled a short Hebrew Bible history.