There are plenty of sources on the internet to get high quality GPS track material from, paid and free. Just be aware that it requires a lot of effort, time and money to scout, create and maintain tracks for off-road motorcycle navigation, so please consider supporting the people and organizations that make this possible. Most sites that offer free track material accept donations to keep up their amazing service.
Below are some sources that I have found or used in the past in different regions.
North America
OneManTrail (OMT) – Incredible collection of GPS tracks all over North America, including the famous Trans America Trail (TAT) which is a coast to coast ride with 60 sections and over 6,000 of awesome off-road adventure, and a 9,800 miles ride through Canada called the Trans Canada Atlantic Trail (TCAT). – FREE

TransAmTrail – The original Trans America Trail (TAT) – 8-30 USD per track, 100 USD for the whole set
GPS Kevin – Another version of the Trans America Trail (TAT) – 125 USD
Backcountry Discovery Routes (BRD) – An awesome collection of high quality GPS tracks all over North America. – FREE
Australia
Trans Australia Trail – A Collection of off-road tracks for motorcycling in Australia. – FREE

Asia
Mongolia – Cheketours published a great collection of GPS tracks for off-road motorcycling in Mongolia – FREE

Vietnam – Tigit Motorbikes, the leading rental shop and expert guide for off-road motorcycle navigation in Vietnam published a collection of high quality and free GPS tracks downloadable from their website for both North and South of Vietnam for all riding levels. – FREE

Europe
Trans Euro Trail (TET) – A web of well maintained and regularly updated GPS tracks all over Europe, built by an amazing community. – FREE

Worldwide
ADV Tracks – Global platform with thousands of off-road tracks shared by community members. A great resource to explore new off-road motorcycle trails. – FREE

Wikiloc – Global outdoor navigation platform. Plenty of off-road motorcycle tracks available, many with pictures added so you can get a better idea of the area. – FREE

Alltrails – Another great source for hiking or motorcycle off-road GPS tracks from other community members. – FREE

Komoot – Global platform for sharing hiking, cycling or running routes. Includes a route planner as well. Another fantastic resource to discover off-road trails in any area. – FREE

Need more information on how to use the downloaded files?
Check out our in-depth guide on how to use GPX and KML files for off-road navigation