Use %in%
for inclusion or negation of a logical vector formed with it to do exclusion:
uk <- subset(x=gtd, country_txt %in% c("Great Britain", "Northern Ireland") )
To exclude members of that set:
not_uk <- subset(x=gtd, !country_txt %in% c("Great Britain", "Northern Ireland") )
Can also do that with [
uk <- gtk[ gtd$country_txt %in% c("Great Britain", "Northern Ireland") , ]