Surprised your having trouble with nickles on your GT, because I find I'm digging more nickles than ever with it. They're easy thanks to the high VDI resolution it has in the foil range (starts at about 60) all the way up to copper pennies, which of course read 180. Nickles will usually read around 142 to 146 for me (using noise band 2 on the GT as band 1 can make them read a bit different as it adjusts mid range target numbers a bit...all the charts use band 2). Anyway, they'll either lock onto 1 or maybe at the most 2 VDI #s no matter which way you sweep over them, and of course they'll have a nice smooth "round" sound to them like a gold ring would too. If the VDI changes by say 3 or more digits as you sweep at different angles then it's probably going to be junk, especially oddly shaped junk. Pulltabs will read around 149 to 169 on the VDI, so it's real easy to avoid those unless they are cut in half badly or something. In fact, most identical tabs I find will read the same ID # as each other even if one is bent or crushed a good bit compared to an identical one that isn't. I'm finding tons of nickles 4 or 5" deep at pounded out sites, indicating people are hitting those spots but not digging the nickles because they might not be able to tell them from other things on their VDI. I've dug my share of war nickles, Vs, and buffalos with the GT too. Just waiting on that first shield nickle.
Just remember, a VDI of around 142 to 146 (most probably read 144, 145, or 146...can't remember which two exactly right now are the most common), and the VDI should lock onto one or at the most maybe two numbers no matter which way you sweep over it. If it changes by 3 or more chances are it's trash. The audio should sound nice and "round" and smooth too. If it does, and the VDI is 1 or 2 digits, then I'll bet it's a nickle every time and it usually is.
Some old nickles can read as low as like 136 on the chart depending on the soil they've been soaking in and how bad of shape they are in, but then I've dug my share of old Vs or buffalos in pretty bad shape that still read right in that 144 to 146 or so range, maybe a few at 142 or 143.