How to deal with unwanted physical contact, presents and attention by a postdoc supervisor?
This might be more of a Interpersonal Skills question, but:

don't take any action on your own
do support your GF in any way she asks, including (1) being supportive and helping her deal with the stress (2) being present as a third party at meetings or (3) in conversation as a reminder that she's unavailable or (4) as an excuse (&quot;oh, sorry, I have plans with my boyfriend (cough) so it'll be inconvenient to meet in person this weekend, but we could communicate via Zoom/Slack/e-mail ...&quot;) — not that it's OK that any of these are necessary in the situation.
after the submission deadline passes, it's a judgement call (depending on personalities, relationships, etc.) whether it's best for her to try to work things out with the postdoc supervisor or go immediately to the professor

@Buffy correctly points out in a comment (I should have included this) that the highest priority is for your GF to stay safe by making sure never to be in a private/one-on-one meeting with the postdoc: if you're not available or she doesn't want you there and she can't set the meeting for a public professional space, ask her to ask a friend or colleague (especially female) to be there (mentioning this advice to your GF would be the one possible exception to &quot;don't give her any help she doesn't want&quot;). If your GF and the postdoc need to work together to take measurements on some equipment, do whatever you have/she has to to make sure there's a third person present.