GAP43 is expected to show at the 43KDa marker so one could assume that it could be your 39KDa band. Unfortunately without a proper control, such as antibody incubated with the control peptide, KO of GAP43, etc it is difficult to be completely sure. It could be that your protein has post translational modifications and what you see are different forms of GAP43. A quick literature search should give you all the information you need.
Hi Ezequiel. Thanks for your answer. The antibody datasheet shows different band profiles for different samples (brain and cell line). I chat with technical support and was advised to consider the one around 55Kda as GAP43.