When I was doing A Level Biology I didn't really see any correlation between how people did in Biology GCSE and Biology A Level and from what I have seen what that person told you is the best indicator at how well people will do at A Level.
I think a lot of students take A Level Biology because they did well at GCSE and think they can achieve the same result at A Level doing the same amount of work as they did for their GCSE. There is a lot of content and you have to be motivated enough to learn it. Everyone I know who tried tended to do well in the exams, while the people who took it just to have an 'easy' science A Level tended to do badly.
So, no, doing well in GCSE Biology doesn't guarantee a good result at A Level, but as long are you're motivated enough there is no reason you can't do well. Hope this helps!