The Rode Videomic is nice. One of the only issues is that you can't monitor the sound.
If you wanted to get a H4n or an H1n, you can use it to dual record and even set up the cable to monitor sound with headphones. You would need a rig to set it all up though.
Juicedlink has some products that could work as well. I'm really interested in their Little Darling which could do well with the filmmaking that I am producing. I'm not sure if that's what you are looking for as well. With those you have to sync sound in post and is best used for body mics.
With your gopro, can you adjust the recording level on the mic input? I can with the DSLRs that I use but I don't use gopros. If so, the Rode will work. If not look at the Zoom products. The H1n can be found for $70-80 and can work as a camera mic or a handheld digital recorder...or both!