Actually if you know what format the gerbers are and if they are of RS274X and from Allegro then its very easy to import them. Just goto File>Import>Artwork and then select the format, Layer to import and units, They will get imported just fine.
If the gerbers are of D format i,e the old format with the aperature file then there could be some issue, because you have to properly create the aperature file and also have a art_param.txt created for appropriate format and then try to import...