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.
(A lot of the StB guys decided to simply take advantage of the coming economical transformation and subsequently stole a lot of corporations and other stuff from the general public)
My point is, the party/StB/KGB calculated (at least in Czechoslovakia - there are their own meeting notes about it) they wouldn't be able to win, and so decided they would rather steal some stuff than destroy it (or let the rebels destroy it). If they wanted to win by force - and initially they wanted to, until they understood the scale of the rebellion - they would need much bigger forces, but that was impossible to arrange, and not for lack of willingness.
They actually deployed the army (both CzSk and Soviet troops) and police, but the rebelling masses were way too overwhelming and they chickened out - there are videos of them going/rolling tanks backwards away from the enormous masses of people (easily hundreds per one troop, not even possible to kill them with your Kalashnikov). If they fired the tank, they wouldn't last 5 minutes and their death would've been nasty. What changed from 1968 in CzSk and from the situation in China was the scale of the rebellion - in 1968 only tens of thousands of people protested, in 1989 it was millions, and each incident (not like there weren't any) caused much more people to rebel instead of suppressing it.
And there was no silence out of Moscow. There are records of USSR-CzSk phone calls about this, and it was definitely not silence nor "we won't do it because it's bad" but "we can't do it because we're out of money, you're on your own BUT DO SOMETHING OR ELSE" (the CzSk side was asking for air support and so on). I assure you that if they went with force all the communist party members, StB/KGB agents and Russian troops would've been hanged afterwards - so the only remaining option was to let it happen and this way they all survived and some even thrived (like for example Andrej Babiš).
Gorbachev's "democratic-communist" (hah, what an oxymoron) Soviet Union could've never worked without the satellite states which supplied most high technology, a lot of important natural resources, etc with dictated prices USSR could afford, so I really don't think he'd just let it go if he had any other option. Certainly the phone calls didn't sound like they were happy about it or that it was the plan all along.
Overall, I don't buy the Gorbachev==good view. Perhaps he let the fall in DDR happen, but probably only because he was busy trying to keep other parts of the empire such as Hungary and the Baltics. He might've been better than the previous leaders, sure - but who isn't better than Stalin and the party? He was still an imperialist even to the modern times, and wanted to force communism (and all the associated baggage) down the throats of his subjects even though the people were demanding the end of planned economy. That makes him bad in my eyes regardless of whether he decided to let the fall happen or not, anyways. Certainly nobody to be fond of, and probably just a case of being at the right time at the right place - it's not like he could've said anything else than he said without being thrown out of the window.
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.