Abdul Qadir Gilani known as Ghaus-e-Azam was a sixth century mystic and a narrator who was born in the north of Iran and died in Baghdad. He was the figurehead of the Qadiri Sufi order. Historically, he lived very much after the period of Imam Sadiq (a.s.). Therefore, the allegation that he opposed Imam Sadiq (a.s) is untrue and baseless.
Abdul-Qadir ibn Abi Salih Musa ibn Abdullah ibn Al-Gilani titled Muhyiddin (the Reviver of the Religion) was born in the north of Iran. He grew up in Baghdad and was buried there. According to Qamoos al-A'laam wa Tabaqat of Sha'rani, Abdul Qadir Gilani was a descendant of Imam Hasan Mujtaba (a.s), thus he was a Hasani Sayed. He is one of the sixth century mystics and a controversial personality of the Islamic world. He was born on a Wednesday the 10th Rabi at-Thani in 471 AH, 1077 AD, and died on Saturday night 1166 (8th Rabi' al-Awwal 561AH) at the age of ninety years (by the Islamic calendar), and was entombed in a shrine within his Madrassa in Baghdad.
At the youth age he went to Baghdad (488 A.H.), where he pursued the study of Arabic with Abu Zakariya Razi. Al-Gilani received lessons on Hadith and Fiqh from Abu Sa'd al-Mubarak bin Ali bin al-Hussain al-Mukharrimi al-Baghdadi. He also learned the mystical order from him.
Some works in jurisprudence and legal theories are ascribed to him namel: "Bashaerul Khairat (Glad-tidings of Good Deeds)"; "Al-Ghunya li-talibi tariq al-haqq wa al-din (Sufficient Provision for Seekers of the Path of Truth and Religion)", "Al-Fath ar-Rabbani (The Sublime Revelation)", "Malfuzat (Utterances)", "Futuh al-Ghaib (Revelations of the Unseen)", "Al-Fuyuzat al-Rabbaniyah fil Awrad al-Qaderiyah", "Adab al-Suluk wat-Tawassul ela manzil al-Muluk" and a "Collection" of his poems which is known as "A Collection of Ghaus-e A'azam's Poems". 
Ibn-e Arabi has made mention of Abdul Qadir Gilani's name in many of his works with respect and dignity.
His Sufi order named after him is generally thought to be one of the most renowned Sufi orders of the Islamic world. He himself enjoyed a special place in the chain of Sufi orders.
Finally, in reply to your question about his relation with Imam Sadiq (a.s.), we must say that historically there is a huge time gap between him and the Imam; he lived very much after the period of the Imam (a.s.). Therefore, the allegation that he opposed Imam Sadiq (a.s) is untrue and baseless.
 - Ali Akbar Dehkhuda, Dehkhuda Dictionary, Vol.10, (Shams Abad, Ali), p.15713, Tehran University Printing and Publication Institute, 2nd edition of new edition, 1998.
 - Martyr Mutahhari Collection of Works, vol.14, pg.570, vol.23, pg.57, Noor Software Program
 -Dehkhuda Dictonary, vol.10 (Shams Abad Ali) p.15713.
 - Muhammad Ma'sum Shirazi, Taraequl Haqeq, (edited by Muhammad Ja'far Mahboob), vol.2, pg.32, Sinai Publications, 2nd edition.
 - Abdul Qadir Gilani, Futuhul Ghaib, pg.10, Darul Hadi Publications, Beirut, 1428 A.H. – 2007.
 - Dehkhuda Dictionary, vol.10, (Shams Abad ali), pg.15713.
 - Futuhul Ghaib, pg.10. ibid.
 - Taraequl Haqaeq, vol.2, pg.362, ibid.
 - Dehkhuda dictionary, vol.10 (Shams Abad Ali) p.15713.