The page you are linking to has a list of abbreviations (Abkürzungsverzeichnis) in which some abbreviations are explained, e.g.
Ln. - Leinen (linen)
Hln. - Halbleinen (semi-linen*)
SU - Schutzumschlag (dust jacket)
kart. - kartoniert (bound in boards*)
Another (extended) list of abbreviations can be found here.
Unfortunately none of these lists says anything about Gln. . But lucky for us this page says
rotes Ganzleinen / Leinen / GLn / Ln kl. 8vo im Format 10 x 18 cm
Therefore we can assume Gln. stands for
Gln. - Ganzleinen (full cloth/pure linen*)
- I am not 100% sure that these are the correct English terms. dicct.cc translates "kartoniert" as paperback (Taschenbuch/Softcover), but usually kartoniert is used when talking about books with a "hard cover".
8 TITEL, dabei Georg Kolbe; Jakob Wilhelm Fehrle; Wilhelm Lehmbruck; Das Buch vom großen Treck; Finnische Kunst etc., Stuttgart u.a. 1922-1949, ges. ca. 960 S., zahlr. Abb., Ln./HLn., tls. berieben u. fleckig, 1xSU, dieser beschädigt <1105158F
could fit.antiquarische Abkürzungen
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