Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!
gwîl
noun. cessation, quiescence, leaving off; quiet, rest, peace
gwil-
verb. to sail, float, fly
A verb appearing in the Gnomish Lexicon of the 1910s as G. gwil- “sail, float, fly” derived from ᴱ√gu̯il (GL/45). It was cognate to ᴱQ. vil- or ’wili- “sail, fly, float” (QL/104).
Neo-Sindarin: Since [ᴹQ.] vil- or wil- “fly” survived in Tolkien’s later writings (Ety/WIL), I would retain ᴺS. gwil- as well, but only for the sense “to fly”. For “float” and “sail” I would use other verbs like ᴺS. loda- “float” [N. lhoda-] and ᴺS. renia- “sail” [N. rhenia-].
gwel-
verb. to boil; to bubble (intr.)
A verb appearing as G. gwel- in the Gnomish Lexicon of the 1910s with the glosses “boil” and “bubble (intr.)” (GL/44). It is clearly based on the early root {ᴱ√WELE >>} ᴱ√GWELE from the contemporaneous Qenya Lexicon (QL/103).
Neo-Sindarin: Since we have no better “boil” words, I retain ᴺS. gwel- “to boil, bubble” for purposes of Neo-Sindarin, but assume it is derived from a Neo-Root ᴺ√WEL so that it retains the same form.
A word in the Gnomish Lexicon of the 1910s said to be used like gwilm “quiet, rest, peace”, but also meaning “cessation, quiescence, leaving off” (GL/45). It was derived from primitive ᴱ✶gu̯ild- base on the early root ᴱ√gwil-.