When Gawain said he wanted to take a look at the Great Barrier, it was obvious that he didn't mean to see from the sentinel tower at the borders of the Gondor Wastelands.
The nearest sentinel tower was located at the end of the plains by the southern foot of the Dark Range. In order to get there, he would have to cross this mountain range and traverse dozens of miles of wasteland. It was something that shouldn't be considered right now.
