There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
gada-
verb. to join, connect, unite
gad(a)
noun. joint, link; small lane, track; isthmus
A noun appearing as gad or gada in the Gnomish Lexicon of the 1910s based on the early root ᴱ√yat- [ẎATA], having several meanings: “joint, link; small lane, track; isthmus” (GL/36).
arthaid-
verb. to join
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
A verb appearing as gada- “join, connect, unite” in the Gnomish Lexicon of the 1910s based on the early root ᴱ√yat- [ẎATA] (GL/36), with the Gnomish sound change of initial y becoming g.
Neo-Sindarin: The later form of this root is ᴹ√YAT or √YAN, so I would update this verb to ᴺS. ianna- “to join, connect” for purposes of Neo-Sindarin, inspired by the neologism ᴺS. goianna- “join with, add to”. For “unite” I would instead use S. ertha-.