Update: Ananya has already come up with a reasonable solution (attach a PDF from Google doc to email), but here's my advice if anybody else runs into the same issue.
Ananya Maiti writes:
Google is accepting proposal responses in a google form this time and is asking to add the proposal content in the description field of the google form  .
Fix up your Google Docs proposal, and include a URL to it in the form. Then if there's time before the deadline, tighten up the form's description field so that it doesn't look like you left things "TO DO LATER".
I updated my proposal after initial review and then got a "Response is too large" error while submitting, I have emailed gsod-support and also posted in their slack channel. But still didn't get any reply.
This is their first year, and it's a new webapp. We're just going to have to be patient with them.
if I can omit sections which are not highly important like Personal Information, Resume links, Reason why I am apt since quite many of these information is available in the form responses ?
Yeah, we didn't know what was available, there's no documentation without pretending you're an applicant and going through the forms, which I wasn't comfortable doing.
Idea: They could have a "guest" account, which doesn't allow you to save (and if you try, tells you to make your own account and provides a button to do that), but allows you to go through the screens.