Gravenberch, a deadline day arrival from Bayern Munich, struck in the 44th minute when the midfielder followed up after Union goalkeeper Anthony Moris spilled a long-range shot from Trent Alexander-Arnold.
But Liverpool made hard work of their stubborn Belgian opponents and it wasn’t until Diogo Jota scored in second-half stoppage time that the victory was secured. The Portuguese forward took advantage of a counter-attack and calmly rolled the ball past Moris from inside the box.
Liverpool manager Jurgen Klopp admitted his team could have done better.
“We lost rhythm slightly and gave too many set-pieces away,” he told TNT Sport.
“Should have had more direction and all these kinds of things, but you take what you can get in these moments and I thought we were really mature and professional but we could have been a lot better in a lot of moments.”
With two wins from two, following their opening victory at LASK Linz, Liverpool top Group E with six points. Toulouse are second with four points after a first-half goal by Gabriel Suazo gave them a 1-0 win against LASK.
Reuters contributed to this report.