I'm pretty sure they are both KM, website shows 61477.2 and app shows 35544? Both the app and website show the same number of logs too?
When I look at your account in US units I see 38200 miles. Switching to KM/L and I see 61477.
Both Fuelly and GasTracker have 2 levels of Settings, "display units" and then "tracking units". Display is what you see when you look at your profile (and others if on the web) and tracking is what you enter into the vehicle logs when adding a fuel up.
It sounds like you likely have your tracking units set properly, but on GasTracker, you need to adjust your Display Units. This can be done by opening the left side menu, tapping Settings, Units, and selecting the appropriate option.
Best if you match what you have selected on your Fuelly profile.
Display units on Fuelly.com-