It looks like wherever they're storing that stuff is in what's known as a serious but stable condtion. Serious as in it doesn't work, but stable inasmuch as it's consistent in not working. Maybe that server is buried deep in the basement somewhere and hasn't actually been maintained since 1995.
You don't get much information when accessing it through a browser but the command line output is a solid indication that the storage medium is on its last legs. The large amount of time it takes to generate a file listing (large even for the normally slow-ish FTP) is the first clue but the below transcript seems to show that the media is corrupt or flaky in some way:
pax@paxbox$ ftp -p ftp.informatik.uni-oldenburg.de
Connected to ftp.informatik.uni-oldenburg.de.
220-
FTP.Informatik.Uni-Oldenburg.DE
Moin!
220 ftp.Informatik.Uni-Oldenburg.DE FTP server (FreeBSD) ready.
Name (ftp.informatik.uni-oldenburg.de:pax): anonymous
331 Guest login ok, type your name as password.
Password:
230-
230 Guest login ok, access restrictions apply.
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> cd /pub/grafik
250 CWD command successful.
ftp> ls
227 Entering Passive Mode
150 Opening ASCII mode data connection for '/bin/ls'.
total 3
drwxr-xr-x 2 ingo 14 1536 Jun 5 1996 format-docs
drwxr-xr-x 2 ingo 14 512 Jun 30 2001 jpeg
drwxr-xr-x 4 ingo 14 512 Aug 29 1995 netpbm
drwxr-xr-x 6 ingo 14 512 Jun 30 2001 png
-rw-r--r-- 1 ingo 14 59523 Aug 29 1995 repGL-0.2.tar.gz
drwxr-xr-x 2 ingo 14 512 Jul 25 1996 seminar
drwxr-xr-x 2 ingo 14 512 Aug 29 1995 test-images
-rw-r--r-- 1 ingo 14 573908 Jun 5 1996 tiff-v3.4-tar.gz
-rw-r--r-- 1 ingo 14 673234 Nov 24 1994 urt-3.1.tar.gz
226 Transfer complete.
ftp> get repGL-0.2.tar.gz
local: repGL-0.2.tar.gz remote: repGL-0.2.tar.gz
227 Entering Passive Mode
150 Opening BINARY mode data connection for 'repGL-0.2.tar.gz' (59523 bytes).
551 Error on input file: Input/output error.
In terms of getting access, I can see two immediate possibilities. The first is to contact the university itself. The second is to try and locate the author, who seems to be one Michael Krause. The author is proving difficult to find using my normal methods (though I'm not the NSA) so I suspect the university may be the more useful approach.
Alternatively, a bit of dedicated research by one Alex Hajnal has turned up this link, which purports to be "an Open Source drop-in replacement for IrisGL using OpenGL under X11". That may well be suitable for your needs if the original cannot be found.
kremvax) – Alex Hajnal Nov 12 '18 at 01:46repGL.tar.gz. One might be able to derive the original Unix sources if you can track that down. – Alex Hajnal Nov 12 '18 at 19:27