I've been there. I've dealt with a guardian ad litem who didn't seem to be able to tell the truth.
My advice I'd give to anybody who has to be interviewed by a guardian ad litem would be to record all conversations between themselves and the GAL. Obviously that depends on the laws in your jurisdiction. I really think that attorneys should insist that all conversations their clients have with the GAL be recorded. Otherwise, the GAL can make up whatever story she wants. And I've seen that they can easily get away with doing just that.
The GAL assigned to my divorce case made false statements to the court regarding my opinions or statements I made.
If a guardian ad litem lies about you (and some do lie), you're going to have a hard time getting a judge to believe your word versus the GAL's.
Oh, and when I asked the GAL assigned to my case why she attributed positions to me in her GAL report which confliced with what I told her, her answer was (and I did happend to record this conversation), "That's just the feeling that I got." The feeling I got, folks. That's what led a GAL to attribute a position to me.