datafr[c(1:3),1]

[1] "C:\Users\bla\foo\bla\foo\Buffer_1000m_arene_full_DUSAF_1980R_300m_BG_1.tif"
[2] "C:\Users\bla\foo\bla\foo\Buffer_1000m_arene_full_DUSAF_1980R_300m_BG_2.tif"
[3] "C:\Users\bla\foo\bla\foo\Buffer_1000m_arene_full_DUSAF_1980R_300m_BG_3.tif"

datafr[,5] <- gsub(pattern='C:\Users\bla\foo\bla\foo\Buffer_1000m_arene_full_DUSAF_1980R_300m_',
                 replacement='',
                 x=datafr[,1])

datafr[c(1:3),5]

[1] "C:\Users\bla\foo\bla\foo\Buffer_1000m_arene_full_DUSAF_1980R_300m_BG_1.tif"
[2] "C:\Users\bla\foo\bla\foo\Buffer_1000m_arene_full_DUSAF_1980R_300m_BG_2.tif"
[3] "C:\Users\bla\foo\bla\foo\Buffer_1000m_arene_full_DUSAF_1980R_300m_BG_3.tif"

Error: 'U' used without hex digits in character string starting "'C:U"