What would you call an operation you can undo?

When you undo an operation in Gmail, for instance, the state of the mail program is reverted by reversing the most recent user action. Therefore, one suggestion to describe your "undo-able" set of operations is:

Reversible — M-W

able to be changed back to an earlier or original state
"Fortunately, the damage is reversible."

_{or revertible — Wiktionary}

You said:

I have used an undoable operation term but then I realized that undoable really means an operation that can not be done.

Interestingly enough, quite a few online dictionaries actually mention undoable in its "can be undone" meaning — Your Dictionary, Collins, TFD, Wiktionary

In some cases the context will be able to tell which version of undoable you're using, but in others, like the one you've quoted, you'll have to use an alternative (and I think that NVZ's reversible is a nice one).

Edit after comment:

Unundoable - Yourdictionary, Wiktionary, Wordnik, Google Books

•Not undoable (unable to be undone).

To add to the list of "re-" words:

revocable

meaning "capable of being revoked or cancelled."

This also has the neat antonym of "irrevocable" for those changes which can't be cancelled.

In the world of Information technology, such thing is called rollback

Rollback

the act of reversing or undoing something

The word was originally use as a method in database management system (DBMS) which has been adopted by some IT professionals in the field as a general terminology in the world of Information Technology.

In the VFX and CGI fields, we call something that you can undo "non-destructive", as in: This program lets us use a non-destructive workflow.

This is in contrast to destructive, where actions are permanent or semi-permanent. While non-destructive is common in those fields, and I believe it also sees use in programming and audio fields, it's not commonly used in regular language. However, it is a very accurate term, and in my opinion at least, is intuitive, when I first heard the term, I never had to ask what it meant.