postcss([require('postcss-transform-css-var')]).process(yourCSS);

const gulp = require('gulp');
const postcss = require('gulp-postcss');
const syntax = require('postcss-less');
const varConvert = require('postcss-transform-css-var');
const through = require("through2");

const convertPipe = () =>
  through.obj(async(file, enc, cb) => {
    let result = await postcss([varConvert()]).process(file.contents, {
      from: undefined,
      syntax: syntax
    });
    file.contents = Buffer.from(result.css, enc);
    cb(null, file);
  })

gulp.src('src/**/*.less', { basePath: './' })
  .pipe(convertPipe())
  .pipe(gulp.dest('./', { overwrite: true }))