In some accents of British English, a word-final /t/ sound after a short vowel can be pronounced as something that sounds like an /r/ sound when the following word starts with a vowel. This is discussed on John Wells's phonetic blog; I quoted the relevant portions in a previous answer.
"Geroff" is a respelling of the pronunciation of "get off" in such an accent. (I don't know whether the specific form "geroff" might have spread to other speakers who don't have this as a usual feature of their accent.)
The British English "t-to-r" sound change is somewhat similar to the American English phenomenon of "flapped" or "tapped" /t/, but also different in several ways. Wells suggests that "t-to-r" developed by way of /t/ leniting in this position to a voiced flap [ɾ], which was then perceived as /r/, causing it to to be subsequently replaced for some speakers with the now more common [ɹ] allophone of /r/ (just as in words like "very", the [ɾ] allophone of /r/ seems to have become less frequent compared to the [ɹ] allophone).
American English speakers perceive a "flapped/tapped" /t/ as /d/, never (to my knowledge) as /r/. And American English speakers seem to use flap /t/ in more positions: inside of words, and after long vowels/diphthongs. So an American would be more likely to respell "Get off" as "Ged(d)off". Also, from the comments section of the John Wells blog post, it sounds like some British English speakers might have both a t-to-r rule and a separate flapping rule.