On dry gravel and dry well packed earth a knobby tread is not always needed and a wide slick is safe to use. If the gravel is wet, smaller knobs are good to have. If the earth trail is wet, a cyclist will need large block knobs and these roll very slowly on firm surfaces.

Vittoria Voyager Hyper: http://www.vittoria.com/product/touring/
150tpi, slick tread, flat protection.

Vittoria Cross XN Pro: http://www.vittoria.com/en/product/c...s/#product-118
150tpi, File tread

Panaracer Pasela: http://www.panaracer.com/urban.php
66 TPI, touring tread with tan sidewalls

Kenda Happy Medium: http://www.kendausa.com/en/home/bicy...py-medium.aspx
120 TPI, Diamond center section with knobby shoulder

Kenda Small Block 8: http://www.kendausa.com/en/home/bicy...ock-eight.aspx
120 TPI, small block tread

Clement LAX clincer: http://clementcycling.com/88/las-clincher
120 TPI, File tread

Challenge Grifo XS 32 Open: http://www.challengetech.it/products...xs-open-020/en
260 TPI, File tread

Challenge L'Eroica: http://www.challengetech.it/products/road/eroica-027/en
260 TPI, textured tread