Look at Ukraine today - they are using so much Western technology, relying on so much Western money - and yet they can't get 500 km past Ukraine border. They wouldn't reach their own border if they had only their own resources.
You don't need to be able to compete with the US military technology to kill unarmed protesters. T-54s will do just fine.
And you're discounting the strength of millions of super-angry people too much. They would destroy the few tanks with rocks (or molotovs, as illustrated in Ukraine) if they had to.
The case being discussed here, Eastern Germany, was just behind the iron curtain, remember? Soviet troops were at ~300 locations on the GDR territory, ~50 airfields, over 300,000 soldiers, over 4,000 tanks.
For one thing, the problem is your tanks and troops have to be ready all around the country - the protesting people are moving across the state quickly. One day there's a protest in Prague, second day it's in Brno - but you can't move your 300k troops and 5000 tanks from Prague to Brno in a day. And then the next day it's Ostrava and you have to do it again. Then an incident happens and that provokes a 10x bigger protest in Prague, Brno and Ostrava at the same time. That's impossible. You need much, much more troops and tanks to handle this scale of rebellion - and the requested air support that never came. And your tanks will never make people go back to work, anyways.
(I'm discussing Gorbachev, not GDR specifically)
I don't know how old you are or where you were at the time. I was there. In the GDR, in East Berlin. On the streets. And I can tell you, a few tanks and troops getting their guns out would have made major impressions on people.
It's not just a numbers game. You are greatly oversimplifying history here. Quite naively so, I might add.
It's a great achievement of history that Gorbachev made the Soviets keep their feet still and among many eastern Germans it's regarded as quite the miracle that this whole episode went down non-violently. Look around in the world in the last decades. This was the major exception, and Gorbachev was central to that.
Also, let's get the picture of the situation straight. He didn't just passively sit bunkered in in Moscow, letting things happen. He actively went out to meet leaders of other involved powers, including the German chancellor and foreign minister, Kohl and Genscher, which he outlived by a few years.