It depends upon several factors, what licence are you being tested for, when did he fly with you, what did he do whilst flying with you and how many hours instruction have you received. As you've mentiond a GST you must be doing a NPPL where the JAA rules don't apply so you could be OK.
JAR-FCL prohibits the test being conducted by an examiner who has given instruction for that rating. You would therefore have to have received the minimum mandatory instruction on the course conducted by another instructor. The CFI may have conducted a progress check but should not have given instruction or be included in the minimum dual total. If he instructed you over 25 hours ago (the minimum PPL mandatory dual) then it should not be a problem otherwise, you will need another examiner.