he's in a hurry

It seems like Gothmog is in a hurry to conquer the city. It takes a long time to move Grond into position and break the gate down. It's daylight when they first show Grond being moved toward the gate, and it's night when they finally break the gate open. They have over 200,000 orcs (according to Peter Jackson) so It seems Gothmog would rather sacrifice a few hundred Orcs in order to save a few hours and make the battle go quicker. This nitpick isn't really a nitpick, it's just the characters personality. He'd rather get the job done quick than safe.

Tactics

Like any commander, Gothmog hoped that the simpler solution - the smaller ram - would do the job. Grond obviously took a lot of time and work to get into position. And Gothmog doesn't care a whole lot about throwing away orc lives.

I agree. except for one detail...

I am the author of this nitpick, and I agree with the comments, except that when Gothmog says that Grond can do the job, he is actually smiling, if you call it that, at least he is cheerful. If he had been more like: - Oh, no, only Grond can do the job, and it will take a loooooooooong time to get it into position, then I would agree that it was just his personality. On the other hand, Saurons armies have been attacking Gondor earlier, and I think even the fact that they bothered to bring along those makeshift battering-rams, is a nitpick to me. Sauron knows that the city, and it´s gate have been standing for thousands of year, and therefore how solid it is.

he didn't know how strong the walls were

When is it said that Sauron, or hit Orcs, actually attacked Minis Tirith in the past? The Orcs have tried to get past the river and Osgiliath. It is never said that they actually got past the river far enough to attack the main city. I never read the books but we are nitpicking the movie, not the book. In the extended edition of the second movie it is said that Osgiliath was once a great city with music and beauty ect. This seems to mean that the Orcs never got far enough to get into, or past Osgiliath. And when they did Boromir went in and drove them out. How could the orcs make it to the Gates of Minas Tirith, when they can‘t get past Osgiliath. And even if they did a long time ago, it wasn’t with a force as great as they have this time. It’s true that the gates and wall of Minas Tirith have stood for a thousand year, but any orcs that reached the gates probably didn’t live to tell about it. Gothmog might have forgot about Grond. It’s not like they build Grond just for Minas Tirith when they didn’t know how strong the gates were. They had Grond on standby if the small rams didn’t work. What’s the point of bringing it all the way to the front lines if they are not sure they are going to need it.

Orc characters

Firstly, Gothmog is following the orders of the Witch King ie not stopping the attack. Secondly, Gothmog doesn't care about his men as shown when he tells them not to move when huge blocks of buildings are falling on them.