So, GOF is a tool in the field of pathogenesis. Say you've got a strain of something that affects mice or mosquitos- it doesn't necessarily have to be humans. But the transmission or virulence of the bacteria or virus is unpredictable and it's not entirely clear how such a thing would reliable kill 50% of your target organism. If you're an epidemiologist or virologist, tweaking the bacteria or virus by inserting some sort of protein coating, immune factor or antigen to smooth your testing can theoretically provide you with better data in those studies. The GOF tweaks needn't be minimal necessarily, but typically they are, to avoid any suggestion that it could spiral out of control or have the appearance of bioweapons. The thinking is that, we should be really, really careful with infectious pathogens, but shouldn't treat everything as untouchable, unknowable and destroy anyone who suggests we learn how it works, like a bunch of Luddites.
Bioweapon research is strictly about two things: (1) death and (2) delivery. There's a little concern about how to effectively, safely and cheaply keep a warm-base production capacity, but mainly those two things. Then, the research is about (1) changing one or few variables at a time, putting it into a test animal population and measuring lethality over time. The other side (2) is about how to infect the biggest population or widest dispersal, through air, water, food, etc., and whatever device(s) are necessary: nebulizers, dissolving capsules, missiles, canisters, etc. Both tracks are repeated endlessly until the BW capability is considered mature and then either used or threatened for use.
GOF is more innocent than many would have you believe, and it's hard not to take Fauci at face value for his claims that this was all NIH-NIAID / BARDA wanted to fund with the Wuhan Institute of Virology. Without such knowledge, epidemiologists and public health officials will have more difficulty in planning response to epidemics / pandemics. But there is that grey area, where there is overlap where GOF research can definitely be used for BW and is definitely a concern.