フォーマットを関数コール形式にする修正が必要になった。

<% include ./head.ejs %>
↓
<%- include('./head.ejs') %>

# search
<% include( )([\w|\.|\/]*) %>
# replace
<%- include('$2') %>

# file to include 
*.ejs

PluginError [SyntaxError]: Unexpected token '.' in /project/filename.ejs while compiling ejs

If the above error is not helpful, you may want to try EJS-Lint:
https://github.com/RyanZim/EJS-Lint
Or, if you meant to create an async function, pass `async: true` as an option.
    at new Function (<anonymous>)
    at Template.compile (/project/node_modules/gulp-ejs/node_modules/ejs/lib/ejs.js:662:12)
    at Object.compile (/project/node_modules/gulp-ejs/node_modules/ejs/lib/ejs.js:396:16)
    at handleCache (/project/node_modules/gulp-ejs/node_modules/ejs/lib/ejs.js:233:18)
    at Object.exports.render (/project/node_modules/gulp-ejs/node_modules/ejs/lib/ejs.js:423:10)
    at DestroyableTransform._transform (/project/node_modules/gulp-ejs/index.js:24:28)
    at DestroyableTransform.Transform._read (/project/node_modules/gulp-ejs/node_modules/readable-stream/lib/_stream_transform.js:177:10)
    at DestroyableTransform.Transform._write (/project/node_modules/gulp-ejs/node_modules/readable-stream/lib/_stream_transform.js:164:83)
    at doWrite (/project/node_modules/gulp-ejs/node_modules/readable-stream/lib/_stream_writable.js:409:139)
    at writeOrBuffer (/project/node_modules/gulp-ejs/node_modules/readable-stream/lib/_stream_writable.js:398:5) {
  __safety: undefined,
  _stack: undefined,
  plugin: 'gulp-ejs',
  showProperties: true,
  showStack: false,
  fileName: '/project/filename.ejs'
}