I could be wrong ... but the 'gunk' looks like rust mixed with water to me. No doubt there is fuel in there too (hence the smell), but the pasty, smeary part looks like water to me.
I'd guess that you're just seeing the build up in the tank from long disuse (no doubt my project looks the same way) and it will clean up relatively quickly with use. Fuel left in a tank will trap any condensation (dew) that forms in the tank, and keep it from evaporating (until the fuel evaporates anyway).
BTW, if you do want to have the tank reconditioned, I'd recommend talking to the folks at Mac's Radiator in Bellflower. Although I've never had them do a gas tank for me, I've been taking radiators to them for many years and they have always treated me right. Unlike some other shops I've been to, they are happy to work with you and do whatever it is that you need. Walk in with a tank, no problem (but please leave the tank in the bin provided outside so it doesn't drip on the floor /bcforum/images/%%GRAEMLIN_URL%%/grin.gif ) Take them the whole car, they'll happily do the R&R for you. And they LIKE interesting old cars.