You can do lower-level GC leves and still get seals that way. There's no significant penalty for doing so. Also you should get 8 leve allowances per day, so it adds up. Beyond that, I'd echo what other people said: FATEs, roulettes, and especially crafting and gathering hand-ins if you have any interest in that kind of content. The seal rewards start off pathetic (8 seals per item at level 1) but you'll level fast and start getting significant amounts, plus as someone pointed out there are challenge log seal rewards for hand-ins.