comment.lua

local U = require "Comment.utils"
local VISUAL_LINE = U.cmotion["V"]
local min, max = math.min, math.max

require("Comment").setup {
  opleader = {
    -- line = "gc", -- TODO: had to switch these to achieve the ft-lua behaviour wanted, but messes up everything else
    line = "gb",
    -- block = "gb",
    block = "gc",
  },

  ignore = function()
    -- ignore empty lines for lua files
    if vim.bo.filetype == "lua" then
      return "^$"
    end
  end,

  pre_hook = function(ctx)
    -- determine whether to use linewise or blockwise commentstrng (only for linewise visual selections)
    if vim.bo.filetype == "lua" and (ctx.cmotion == VISUAL_LINE) then
      local diff = max(ctx.range.srow, ctx.range.erow) - min(ctx.range.srow, ctx.range.erow)
      ctx.ctype = (diff > 0) and U.ctype.block or U.ctype.line
    end
  end,
}

try this

pre_hook = function(ctx)
    -- determine whether to use linewise or blockwise commentstrng (only for linewise visual selections)
    if vim.bo.filetype == 'lua' and (ctx.cmotion == VISUAL_LINE) then
        local diff = max(ctx.range.srow, ctx.range.erow) - min(ctx.range.srow, ctx.range.erow)
        ctx.ctype = (diff > 0) and U.ctype.block or U.ctype.line
        return require('Comment.ft').calculate(ctx)
    end
end