Praise be to Allah.
If a person does ghusl following janaabah, and intends thereby to remove both major and minor impurity, then that is sufficient, whether he does ghusl before the time for prayer begins or afterwards. See question no. 88066.
If you did ghusl with the intention of removing both major and minor impurity at the time of duha (forenoon) for example, then it is permissible for you to pray Zuhr with the purification, unless it is invalidated by something that invalidates wudoo’, such as passing urine.
As for a drop of urine coming out after doing ghusl, if you are certain about that, then you must do two things:
Wash whatever has gotten onto your clothes or body, and repeat wudoo’ only, not ghusl.
You should not pay any attention to this feeling and you should ignore it, so as to avoid falling into waswasah. You do not have to squeeze your penis until whatever is in it comes out, or inspect it after you have finished relieving yourself, because that opens the door to waswasah, so you should not pay any attention to that.
It says in Minah al-Jaleel: When he has finished urinating and cleaning himself, he should not pay any attention to anything that he thinks may have come out of him and he should not check it again. End quote.
It says in al-Mawsoo’ah al-Fiqhiyyah (4/125):
The Hanafis, Shaafa’is and Maalikis stated that when he has finished washing himself with water, it is mustahabb for him to sprinkle some water over his private part or pants, so as to put an end to waswaas, then if any doubt occurs in his mind, he will think it is because it is because of the water that he sprinkled, unless he becomes certain that it is otherwise. If he thinks that something has come out after he has washed himself, then Ahmad ibn Hanbal said: He should not pay any attention to it unless he is certain, and he should ignore it because it is from the shaytaan, and it (the waswaas) will go away in sha Allaah. End quote.
‘Abd al-Razzaaq narrated in al-Musannaf that Ibn ‘Umar (may Allaah be pleased with him) was asked about that and he said: If you do wudoo’ and sprinkle some water on your private part, then ignore it, because it is from the shaytaan.
Sulaymaan ibn Yasaaar said something similar, as did al-Hasan al-Basri and Sa’eed ibn al-Musayyab.
See: al-Mudawwanah (1/120) and I’laam al-Muwaqqi’een (4/63).
See also question no. 93877.
And Allaah knows best.