From a legal perspective, you have no standing to permit you to participate in the litigation. While either party might call you as a witness at trial, you do not have the legal ability to file pleadings or to otherwise actively participate in the case, this would include appearing in court and asking your own questions.
For what it's worth, you might consider meeting with your girlfriend's attorney in advance. You could express your concerns and the two of you could discuss how your testimony should proceed.
Please let me know if anything requires clarification.
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