Well, then most GMs lack imagination
The fact is, the Core rule Gifts arent that noteworthy is most cases. PCs on the "Road to Glory" are often there because they want power, ie Gifts should sometimes actually be empowering and not just a +10 bonus to a test. And there are so few that are actually inspiring in any way.
Especially when compared to say WFRP2 Tome of Corruption, in that supplement there is rarely any point to making your own mutation since there are probably over a hundred different mutations + Rewards of Chaos.
But of course, GM should be consulted in all custom Gifts. GM is always final arbiter on what could work.