The author of that article, chief legal counsel of the right-wing Judical Crisis Network, is basing her argument against Garland on two cases:
1. NRA v Reno
The Brady Bill directs the feds to run criminal background checks on would-be gun purchasers. To avoid a de-facto national gun registry, though, the checks need to be destroyed after the sale completes. They were originally destroyed immediately after the sale finalized, but Janet Reno changed the rules to retain the checks for 6 months, ostensibly for two reasons: to police the government (so that unauthorized checks against random non gun-purchasers by corrupt officials could be caught) and to guard against gun purchases made under stolen identities.
Garland’s opinion was that if the law required the records destroyed immediately, Congress would have specified a timeframe. Given the ambiguity of the law, had Garland imposed a timeframe on the government he would have been legislating from the bench. So his ruling was the conservative one. After the case, Congress always had the chance to specify a timeframe for destroying the checks but never did. When Ashcroft came in, he canceled the 6 month thing.
2. Heller v DC
This was a challenge to a DC handgun ban. First, a three-judge panel declared the ban illegal due to the 2nd Amendment, upending more than 200 years of jurisprudence. Given the weight of the ruling, Garland, along with three other judges including arch-conservative A. Raymond Randolph and uber-liberal David Tatel, voted for a rehearing with the entire DC circuit weighing in instead of just this 3 judge panel. In a 6-4 vote, a majority of the circuit decided to not rehear the case, and so the panel’s judgement stood. Garland never gave an opinion on the case, he only voted for the entire circuit to rehear it. So did Randolph, but the Judicial Crisis Network lawyer conveniently leaves out this fact. According to the JCN author, one of the three other justices was a liberal, Tatel, and that's all the proof she needs to brand Garland as an anti-gun nut.
So that’s the extent of all this chatter about the 2nd Amendment: how long the government is allowed to hold onto criminal background checks in the absence of clarity from Congress, and Garland's vote that the full circuit and not a 3-judge panel should hear a case with far-flung consequences. All the 2nd Amendment talk is nothing but hyperbole and spin, but what else would you expect from National Review?
BTW, uber-Republican Orrin Hatch advocated for a Garland nomination to the Supreme Court in 2010. Here's what he said just last week:
“The president told me several times he’s going to name a moderate [to fill the court vacancy], but I don’t believe him. [Obama] could easily name Merrick Garland, who is a fine man. He probably won’t do that because this appointment is about the election. So I’m pretty sure he’ll name someone the [liberal Democratic base] wants.”
So even in the words of dyed-in-the-wool Republicans, Garland is a moderate.