I'm not sure how you would objectively measure that risk but you could look at cases where others have already got in trouble for using it. I'm not aware of cases where a font was changed because of its problematic creator.
I do believe it's a matter of time before Gill's work becomes associated with his crimes in a more public way.
Hall (2018) writes:
The debate on Eric Gill is not a new one. We have known for almost 30
years about the assaults described in his own journals. Gill Sans has
only been added to the Microsoft library in the last two. Fiona
McCarthy published her revealing biography in 1989 and the BBC chose
to use Gill Sans for their logo in 1997.
Graphic design educators' responsibility to teach their students responsible typeface choice has been brought up in the field, namely by Dave Gottwald. I started avoiding Gill Sans when I learned about the artist's history in Garfield's Just my Type (2010) and I've been removing it from any assignment I've handed out. So while Gill's crimes are not necessarily public knowledge, they've seemingly been attracting increasing attention in the past decade.
The problem with measuring risk is there would also be multiple factors you should consider such as:
How much visibility is involved?
When we look at recent events related to racial equity in the design world, it's not a coincidence that Uncle Ben's and Aunt Jemima have decided to change their famous branding now. Riley and Katyal (2020) wrote in a recent opinion piece:
Brands can no longer stand apart from social movements and activism.
In order to succeed, they have to personify change — to be the change
— through rebranding themselves, or risk serious criticism.
It seems a lot less likely your client would come under fire for something like a poster for a local event.
To what extent is the visual enmeshed with various heritages where it will be used?
Fonts represent more than a mere product of their creator and this makes decisions much more complicated. You may not be able to predict how history will influence how the design is perceived.
An example would be how blackletter typefaces pretty much disappeared from common use. Smith (1998) writes:
During the Nazi era, Hitler embraced blackletter as authentically
'Volkisch,' thereby tainting a number of contemporary blackletter
typefaces just as several talented German designers were rejuvenating
the tradition with modernized versions. Then, in 1941, Hitler abruptly
reversed himself, banning blackletter entirely: it was hampering
communication with the occupied countries. Contaminated, then exiled,
the blackletter typefaces never recovered.
For example, Gill Sans has been referred to as the "Helvetica of England" (Archer, 2007) and is part of the British visual heritage. While their logo still uses Gill Sans, the BBC decided to switch from Gill Sans to a custom corporate typeface in 2018, stating among others that their choice of typeface "[...] must echo our values whilst expressing itself appropriately for differing content and audiences" (Bailey, 2018). I guess if the BBC ever came under fire for using Gill Sans, they could point to this change as evidence that they were gradually dissociating themselves from the font.
Can you easily avoid using Gill's work or is the type part of a client's styleguide?
The eponymous Gill Sans seems that it would be more problematic. Is Gill Sans really the best font for the job? Would it be worth considering using Gill Sans Nova, a revival by George Ryan? Multiple articles out there suggest beautiful alternatives for Gill Sans.
Gill also created "twelve other typefaces, including the popular classical serifs Perpetua and Joanna, as well as Felicity, Solus, Golden Cockerel, Aries, Jubilee and Bunyan." (Garfield, 2010). Can you find proper alternatives for all of these? How much effort is involved?
Will your client have the resources and foresight to make the right decisions when the time is right?
Can you count on marketing experts and enough data to make the right call when the time comes? Aunt Jemima's brand has been problematic for a long time. In fact, I recall it was mentioned in a design class I took back in 2001. So why is it getting changed now? I expect there is a lot of market research around this change. They can make the change now, looking good while doing it, and getting massive media coverage about it. Who is truly benefiting here?
I'm certain there is more to consider than just those items I've listed above. My answer mostly aims to depict how complex measuring that risk would be.
Equally important to consider is: Is it a risk for you as well, as a designer? Is it ever fine to use and indirectly support Gill's work, knowing what you know?