Interesting: Their (Grimm versus Gormek) specials are identical: Ramming Pulverizer at 295%. They differ on the rest of their stats.
Grimm has more tile damage (thus his special is stronger). As a result, he’s better against titans. Gormek has a little more defense and a lot more heart.
If you leveled Gormek, you could make your defense Sonya - Gormek - Heimdall - Rigard - Li Xiu.
This would put a debuffer on the left wing. I think Gormek is sturdier than Kelile and since you’ve emblemed Rigard, it would be too passive to add Boldtusk to your defense.
Ultimately, I recommend you identify one hero of each element and work on all five of them simultaneously. Thus you don’t have a Grimm versus Gormek decision, but a Gormek versus Scarlett decision (or rather, five of them).