I am not familiar with the gbm.plot function. Could you use code like the example below but substituting gbm.plot() where I have plot() and setting gbm.plot to make only one plot at a time? That may be a crazy suggestion, I admit.
X <- 1:5
Y <- 1:5
par(mfrow = c(1,3))
par(mar = c(8, 4, 4, 2) + 0.1)
plot(X, Y, xlab = "", las = 2)
title(xlab = "First", line = 7)
plot(X, Y, xlab = "", las = 2, col = "red")
title(xlab = "Second", line = 7)
plot(X, Y, xlab = "", las = 2, col = "blue")
title(xlab = "Third", line = 7)

Created on 2020-06-18 by the reprex package (v0.3.0)