A verb appearing as gada- “join, connect, unite” in the Gnomish Lexicon of the 1910s based 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-.
A noun appearing as gad or gada in the Gnomish Lexicon of the 1910s based the early root ᴱ√yat- [ẎATA], having several meanings: “joint, link; small lane, track; isthmus” (GL/36).