You only get gumps once you hit 100 skill. If you miss one gump, you will not get banned. You just get a 5 minute jail sentence and a missed GUMP count (which really only impacts repeat offenders). TL;DR: Get macroing!
I leveled up 3x fishermen to 99.9 from Serps bank. Make sure you add in a <99 hits = say Guards! If you're at 700 skill points, lock an unused skill at .1 to avoid getting jailed while macroing. Good luck!
The macro gump must not come up too much for fishing. I macroed 10+ hours after hitting 100 and never got a gump. However, after 10 minutes of "chain selling" to NPCs (gate to house, restock macro, gate to vendor, spam vendor sell/sell agent), I got a gump. I also noticed they came up pretty quick for lumberjacking. Is it random or is fishing skewed to allow longer fishing before gumps come up?
I thought you could macro on land to GM (without getting the gump) since you can't pull anything of value.