Just crafting (but helps ALL crafting skills that can make armor or weapons - Blacksmith, Carpenter, Fletcher, Tailor, Tinker):
At Grandmaster
+5% DI for Weapons, on top of any Exceptional/Runic+Recipe bonuses.
+5 more random Resists on top of the Exceptional Bonus for armor (20 instead of 15 for non-runic, 11 instead of 6 for runic). This is a real boon for the SP crafters (effectively 30 more resists for a GM suit; 6x47/5 = 56+ average for resists on GM suits, up from just over 50).
I'm one of the people that considers any kind of skill gain that doesn't involve DOING things in game, to be a waste of time. One doesn't get gold or loot from looking at weapons or bashing a golem. I found that, just filling smith BODs, and tailor armor & hat BODs (only works for items with resists or that can cause damage, hence the tailor BOD choices), I gain about 3-5 times in Arms Lore for every 1 crafting skill gain, at any level (and was getting 10-20 gains an hour when was getting maybe ONE doing the "repeated lore an item" method - and that might have been a GGS). So, all my Arms Lore gains have been from filling
BODs (except on my fletcher, who gets them doing bow quests for runics).
If you start a blacksmith or blacksmith/tailor as a standard blacksmith template (non-custom, non-advanced), then do the smith train quests in New Haven, you'll probably hit GM Arms lore about the time you hit GM to 110 Smith - sooner if you have tailor or fletcher, or make staves/crooks to train carp and/or weapons to train tinker on the same character.