expected output

 -- should be cut from here
local alpha
      = FUNCTION ( -- this is NOT my line number
          FUNCTION 'beta' { 'gamma' } ()
            and 'epsilon'
             or 'zeta'
        )
      {
        'eta'
      } -- should be cut from here
print("bar")

gistfile1.lua

local str = [[
print("foo")
local alpha
      = FUNCTION ( -- this is my line number
          FUNCTION 'beta' { 'gamma' } ()
            and 'epsilon'
             or 'zeta'
        )
      {
        'eta'
      } -- should be cut from here above
local alpha
      = FUNCTION ( -- this is NOT my line number
          FUNCTION 'beta' { 'gamma' } ()
            and 'epsilon'
             or 'zeta'
        )
      {
        'eta'
      }
print("bar")
]]

-- enclose string literals and table constructors in temporary parentheses
str = str:gsub('%b""','(\0%0\0)'):gsub("%b''",'(\0%0\0)'):gsub("%b{}",'(\0%0\0)')
repeat
    -- set "chained function call" mark
    local ctr
    str, ctr = str:gsub('(.*)%b()%s*%b()', '%1<\0>')
until ctr == 0
-- search for last "chained function call" mark and remove all text before it
str = str:gsub('^.*<%z>', '')
-- remove temporary parentheses
str = str:gsub('%(%z', ''):gsub('%z%)', '')
print(str)