Well, it is my first language, so I disagree.
I am, however, listening to an audiobook version, so maybe it's different on the page.
I also don't think he's a bad writer or that his Russian is quite that strange (in Archipelago, specifically) but I'd be remiss not to mention his literary style has been had its share of critics, with Vladimir Voinovich putting in what is perhaps the top effort in the genre. On the off chance either of you have not run across it before (in which case, read the whole book) [Симыч here is the parody stand-in for Solzhenitsyn]:
https://www.e-reading.club/chapter.php/11816/25/Voiinovich_-...
For sure. Voynovich may not be quite as profound as Solzhenitsyn but his writings are actually pleasurable (IMO) to read. I've heard ерфе the real Karnavalov was not amused.
It's hard to imagine anyone mentioned there was particularly entertained.
'Invented words' is what made me think of it, I don't think it's much in evidence in GULAG but it definitely pop up in his fiction. In Voinovich's parodic telling, it's due to an excessive reliance on the 19th century 'Dahl's Explanatory Dictionary'. It's a perfectly pointed barb.
I think where GULAG succeeds is in exactly what he puts on the front page "an experiment in literary investigation". Everything from 'В круге первом' through 'Как нам обустроить Россию' and later gets progressively weirder and, well, to put it politely, not better.
I am a speaker of a Slavic language other than Russian, speak some russian, read Gulag Archipelago in English, and found it to flow wonderfully. OTOH, I never liked the style of Dostoyevsky, Tolstoy, and most other Russian classics. So maybe Gulag is not up to Russian literary tradition, but for me that's a good thing :)
Obviously, this is to a large degree a matter of taste, but I am far from the only Russian speaker finding Solzhenitsyn's language highly stilted and artificial. The English translation you're quoting is much better.
Interestingly, same is very often said about Vonnegut's translations into Russian.