When you start to use the word "assume" three times or more, you might be too broad and might get hit by a lot of individual opinions. Anyway…
Does the technique your assumptions describe actually exist?
Yes, it exists. It's part of what's commonly known as "body tracking surveillance techniques".
Will you ever meet it in real life?
Probably not — unless governmental institutions like the NSA learn to love you.
Is eye-tracking the only problem in your scenario?
Based on the assumptions of your scenario descriptions, you should also look into "body tracking" and "statistical identification of movement patterns". To give you a simple hint on these kind of things: "shoulder movement" on it's own (no matter how minimal it is) can already provide enough identification vectors to reproduce the keys you have typed with a high probability of success.
Yet, physical tracking methods are rather impractical on a "one-person" scale as there is too much personel (specialists evaluating the data) and hardware (needed surveillance equipment) involved.
Why would someone want to use complicated techniquest when there are much simpler ways to gain access to your passwords. Let's - just for fun - assume someone really wants to get your password/pin/code/whatever using physical tracking methods… that someone will have to gather additional reference data and - even more important - be sure that the resources invested are actually worth the result.
So, unless you're working for "el presidente" and got some really cool access level to classified information - most adversaries will most probably fall back on easier methods to gain access to your passwords. After all, the goal of "them" will always be to invest a minimum of resources to reach access to your passwords. Trust me, no one will ever hire two to three infosec specialists with hi-tech equipment just to steal your top-secret Gmail password.
But let's suppose your password is really that important. Let me list a few security problems you'll have to take care of first:
- Is that waitress really cleaning that table (or is she watching what you type)?
- Did you check the tables that surround you (and the people sitting near you who could be watching you)?
- Are you sure there are no hidden cameras recording you from intrusive angles?
- Are you sure there is no hidden microphone underneath your table to record your keystrokes (which can aid reconstructing the letters you type using sound-to-microphone distance vectors)?
and last but not least
- Are you really sure you want to use such an important password in a public place like this?
Wrapping it up...
Let's be honest — no one will use eye-tracking when there are plenty of alternatives to get what you think is worth securing. Compared to eye-tracking worries you describe, those alternatives will certainly provide easier options to gain access to your secured data.
But assuming the data you plan to enter in your device is really that confidential, I would take a coffee-to-go and look for a more trusted place to enter that data/password/pin/code/whatever.
In all other cases, it's time for a little reality check to prevent the security-related paranoia that hits us all every now and then…