Are your parents' profiles in the GEDCOM? If so, find one of them. They should be matched to the profiles you've already created on WikiTree for your parents; if not, use the Compare With link to compare and match the person in the GEDCOM to the person on WikiTree.
Once they're matched, click on the Compare button, and you should get a box with the GEDCOM connections on the left and the WikiTree on the right. It'll show the person, father, mother, siblings, spouses, and children. There should be Add buttons by the not-yet-added people on the GEDCOM side; click those to add the parents, spouses, and children (don't add siblings before you've added parents, as that complicates things).