I usually do it with the ape package first:
library(ape)
library(ggtree)
tree <- read.tree(treefile.tre)
tree <- drop.tip(tree, c("Gluconobacter_donii_Alphabacteria_AB178405.1", "Pseudomonas_aeruginosa_AB037545.1", "Paucibacter_toxinivorans_KX085481.1")) #you need to make a vector for the tips to be dropped