`gulp-postcss is not a function` type error when it provided by gulp-load-plugins

I make some kind of mistake when extended TypeScirpt interface IGulpPlugins from type definitions for gulp-load-plugins, or incorrectly defined types of gulp-postcss.

According comment in index.d.ts of @types/gulp-load-plugins:

/**
 * Extend this interface to use Gulp plugins in your gulpfile.js
 */
interface IGulpPlugins {
}

All of below plugins except gulp-postcss works.

import * as gulpDebug from 'gulp-debug';
import * as gulpPlumber from 'gulp-plumber';
import * as gulpIntercept from 'gulp-intercept';
import * as gulpIf from 'gulp-if';
import * as gulpPug from 'gulp-pug';
import * aas gulpHtmlPrettify from 'gulp-html-prettify';
import * as gulpSass from 'gulp-sass';
import * as gulpStylus from 'gulp-stylus';
import * as gulpSourcemaps from 'gulp-sourcemaps';
import * as gulpPostCss from 'gulp-postcss';

export interface IGulpPlugins {
  plumber: typeof gulpPlumber;
  debug: typeof gulpDebug;
  intercept: typeof gulpIntercept;
  if: typeof gulpIf;
  pug: typeof gulpPug;
  htmlPrettify: typeof gulpHtmlPrettify;
  sass: typeof gulpSass;
  stylus: typeof gulpStylus;
  postCss: typeof gulpPostCss;
  sourcemaps: typeof gulpSourcemaps;
}

The are no TypseScript compiling errors in below code, however if to execute compiled TypeScript, TypeError: gulpPlugins.postCss is not a function error will occurs.

import * as gulp from 'gulp';
import * as GulpPlugins from 'gulp-load-plugins';

// I did not create types for this libary yet, so temporary make TypeScript to ignore it
// @ts-ignore
import * as postcssNormalizeWhitespace from 'postcss-normalize-whitespace';

gulpPlugins: IGulpPlugins = GulpPlugins();

return gulp.src(SOURCE_FILES_GLOB_SELECTION)
    .pipe(gulpPlugins.postCss([postcssNormalizeWhitespace]))
    .pipe(gulp.dest('build'));

If to write

import * as gulp from 'gulp';
import * as GulpPlugins from 'gulp-load-plugins';

/* When I wrote this question, types in DefinitelyTyped for this library was not exists yet,
 * but I created it. However did not create the pull request yet.
 */
import * as gulpPostCss from 'gulp-postcss';

// I did not create types for this libary yet, so temporary make typescript ingore it
// @ts-ignore
import * as postcssNormalizeWhitespace from 'postcss-normalize-whitespace';


return gulp.src(SOURCE_FILES_GLOB_SELECTION)
    .pipe(gulpPostCss([postcssNormalizeWhitespace]))
    .pipe(gulp.dest('build'));

instead, JavaScript will work without errors.

I can not understand, where I did mistake. Let me repeat that all plugins in extended IGulpPlugins except gulp-postcss works.

The types for gulp-postcss that I wrote are:

/// <reference types="node"/>
import Vinyl = require('vinyl');

declare function GulpPostCss(plugins?: Array<string>, options?: GulpPostCss.Options): NodeJS.ReadWriteStream;
declare function GulpPostCss(callback?: (file: Vinyl) => { plugins?: Array<string>, options?: GulpPostCss.Options }): NodeJS.ReadWriteStream;

declare namespace GulpPostCss {
    interface Options {
        parser?: string;
    }
}

export = GulpPostCss;

from `gulp-postcss is not a function` type error when it provided by gulp-load-plugins