If I feel a drop of urine come out when I'm performing ghusl janabat in the shower can I continue with my ghusl since I did not watch my lower area yet or did I have to restart?
Note that ghusl is considered as necessary in certain cases: janabah (by intercourse, or if semen comes out), menses, child birth, death, conversion (according to some scholars) etc. (See also islamqa #93027)
While wudu' is necessary in other cases among them is if one pees or mady or wady comes out. For details refer to Which things break the wudu?.
Therefore if you are performing ghusl and peed (no matter if it was intentionally or not) or anything which invalidates wudu' happened to you. You have not invalidated your ghusl as only one of the things mentioned above require ghusl. So your ghusl is correct and valid, but the wudu' is not therefore to be able to pray one needs to perform a new wudu' only (see islamqa #49693 and also islamweb #135504).
I don't know how to translate the answers exactly in English.
But according to the Fatwa in the following link: http://fatwa.islamweb.net/fatwa/index.php?page=showfatwa&Option=FatwaId&Id=135504
You don't have to restart Ghusl, but you need to do Wodou' right after Gyusl in order to be able to perform your prayers.
Of course if what you mentioned is a drop of (Urine)