With 102 marks in JEE 2019, what would be the apx percentile and rank with Gen category ? Would I be eligible for JEE advance ?

You possible rank is around 53k-60k with this rank you wouldn't be able to get NITs or IIITs as for that you need a rank below 25k but you could get admission into other private colleges affiliated to JEE Mains so I would suggest you to either settle for them or try again next year with better preparation. Though the qualifying cutoff for advance isn't out yet but with this rank I think you would be able to get qualified for advance.
Note that this rank is predicted by rank predictor the actual rank may differ than this.
Hope it helps.
Thank you.