You go Gnome to Goblin because Gnome stuff is all trinkets, whereas some Goblin specialties are consumables, like Sapper Charge, Big One and Mortar (6 charges which you then refill with 1 mithril, 1 solid powder and 1 trigger I think).
You can use every Gnome trinket as a Goblin engy as long as your engineer level is high enough, so once you craft every Gnome item, there is no point in staying as Gnome Engy unless you want to fill some weird niche of trying to sell gnome trinkets to other engineers. Only things you can’t use once you change to Goblin are the Gadget transporter, World Enlarger and Lil Smoky pet I believe, and all those are pretty meaningless novelty items.
You could also try to just buy many of the Gnome trinkets off a Gnome engy, so you can go Goblin from the start. However, A LOT of engineers go Goblin from the start already…and the others will go Gnome to build their stuff and then drop it, so you might have some trouble actually finding the ability to buy all the Gnome stuff, your mileage will vary based on server I suppose. A lot of people do not stick with Gnome so it’s your risk if you want to try to find the Gnome items for sale if you go Goblin first.
It’s also really not that bad of a grind. You can easily level up your mining/engy while going 1-60 so you should have all the gnome stuff made by your early 50s up to 60, drop it at like 265 engy when you have it all, then go Goblin with the extra mats you stored on a bank alt. Buy what you might need off the AH. The only expensive/slightly grindy part is getting through the Thorium levels of engineering, which takes roughly 200 thorium bars, which is why you ignore those mostly as a Gnome since the cut off for the trinkets is, I believe, 265 or so.
Going from 270 to 300 is basically you spamming thorium widgets and thorium tubes and no new Gnome items can be made at this level.
And you want to do this before you learn recipes like Thorium Shells, Sniper Scope and the 3% hit scope as those are all hard to obtain or expensive recipes and you will permanently lose them if you drop engineering and level it again, so only learn these once you’re on your second pass with Goblin.
Also about the transporters, the Winterspring one for Goblin is superior because you can teleport to the zone needed for Wintersaber rep, Timbermaw rep, various Warlock quests, Ony attunement parts, one of the best zones for farming Thorium, one of the best zones for farming winter squid and nightfins as well. The Gnome teleporter is only good because it takes you close to Silithus when AQ is released…but who cares, just fly or get summoned by your raid.