Here are the easiest and closest ways to get to Spain or France with your bike from Gravesend.

This link will give you the best roads to ride in 90 different countries.
www.bestbikingroads.com

Note: There are other ports that you can also choose from. 
Click on the Brittany Ferries link listed below to find these ports.

---

Gravesend to Spain

If you wish to travel to Spain, the ferry in Portsmouth is only 160km from where you collect your bike in Gravesend a 2 hour ride.   Prices start from £204 per bike and rider with a 2 birth cabin one way.  Its a 24 hour sailing UK to Santander.‎
www.brittany-ferries.co.uk

     Gravesend to France

If you wish to travel to France, the Euro tunnel in Folkestone is only 75km from where you collect your bike in Gravesend a 1 hour ride.  Prices start from £25 per bike and rider with pillion one way.  It is a 35 minute train ride under the channel to Calais in France.  Doesn't get any easier than that.  Cheap as chips. 
www.eurotunnel.com