I was fired for gross misconduct , and want to apologize to my former mangers as I didn't see them at all during my last days at the company. Should i ring/message them to ask to meet face to face to chat, or will an email/message suffice?
If there is the possibility that the company could sue you or carry out some other form of legal action for whatever it is they fired you for, then it's probable that a lawyer would tell you to not admit anything and not to communicate with your former employers.
Otherwise you make amends by learning from your mistakes and moving on.
If you aren't worried about future consequences of an apology, just email them. Other types of communication (face to face, phonecall etc) could be seen as potentially threatening.
At the end of the day, you don't owe them anything.
You haven't mentioned what "gross misconduct" you were fired for - knowing would make it easier to answer. If your gross misconduct was slapping a co-worker or walking into a site where concealed weapons are prohibited (with one)... asking to meet face to face is more of a problem than if they figured out that you never did the backups you were supposed to do as a part of your job.
If you worked with your former managers long enough, you could offer to buy lunch as an apology/peace offering. Write (or email) a note to each of them saying both that you regret what you did, and that you understand that you should have been let go for it. Tell them that the lunch is a further way of apologizing for embarrassing them.
It is very unlikely that they will accept your offer of lunch - the point is to give them an opportunity to respond to you saying that you don't need to do anything for them, and it is okay.
The point is for you to communicate that you don't harbor any ill feelings toward them. Keeps them from wondering if there is someone out there who hates them. Which is mostly irrelevant, but if they are ever asked about you in the future they're less likely to torpedo your job opportunity if they know you're sorry for screwing up at this job (they know you own it, shows integrity). I would not, however, advise that you use them as a reference in the future.