I did the C&O and GAP last year. Some of this info might be a repeat of others posted here but here are some links and apps I found useful. Note, cell service on the C&O is sporadic.
Great web sites:
https://gaptrail.org/ and
www.bikecando.com
Apps:
GAP app for iPhones only -
GAP Trail
C&O app -
C&O Canal Explorer Mobile App
Be sure to check the National Park Service website before you go for last minute updates:
https://www.nps.gov/choh/planyourvisit/conditions.htm
and mile-by-mile guide to services:
https://www.nps.gov/choh/planyourvis...itled-2018.pdf
Have a great trip!