GR/gr is ISO for Greece, but EU uses EL/el the previous commenter wrote. The latter seems to come from Greek language, although then spelled using the Latin alphabet
How are those cases similar, except for it's not obvious to the average English speaker where some letters came from?
EDIT: Perhaps it would have been nice if ISO 639 and ISO 3166 had been better coordinated so we didn't have cs_CZ, da_DK, sv_SE, ...
But the standard 2-letter code for the Greek language is "el."
Obviously there are many other countries with abbreviations based on English and there's some decision-making related to avoiding conflicts, but it is a little odd. My first guess would be that the ISO country code logic went something like: prefer native names, if they use the Latin alphabet.
But GB is the ISO 3166 code for the UK, including Northern Ireland. The reason is that the ISO committee wanted to avoid generic words like "united" and "kingdom" so they went for the more "unique" GB, in spite of being somewhat politically sensitive, confusing, and inconsistent with some other standards and every-day usage.
[1] https://en.wikipedia.org/wiki/List_of_islands_of_the_British... [2] https://en.wikipedia.org/wiki/Crown_Dependencies