The official website https://gate.iitb.ac.in/faq.php mentions that if you do  not upload your signature, the application will be rejected. So please ensure you are following the guidelines as mentioned:

• Sign with a dark blue or blank in within a space of 2cms height and 7cms width on an A4 paper
• Make sure it is not signed in Capital letters
• Maximum allowed size is 150*560 pixels and the minimum allowed size is 80* 280 pixels
• Get it scanned and then upload
• Make sure its in JPEG format

If you have submitted your application form without uploading the signature, your application might be rejected and so you have to re-register and fill another application form.