It can be difficult to find vegetarian, vegan and gluten free restaurants in the centre of Frankfurt so I've listed some of the places that I like, which offer something for alternative appetites.
I've used the following markers at the beginning of each restaurant for easy identification:
V = Vegetarian
VG = Vegan
GF = Gluten free
Near the main station (Hauptbahnhof) and Willy-Brandt-Platz
V - Saravanaa Bhavan (Kaiserstr. 66) is 100% vegetarian and serves the best Indian food in town. Try the dosa, it's a good as any in India! Close the main station in the Bahnhofsviertal.
V - Urban Kitchen (Kaiserstr 53 & Börsenplatz 14) with two centrally located restaurants in the city, Urban Kitchen is a little formulaic but offers a choice of dishes, e.g. pizza, noodles, salads and it clearly identifies vegetarian options on the menu.
V, VG & other allergens - Kaiserzeit (Kaiserstr. 59) close to the main station is an organic bakery which opens early (7am) to cater for the breakfast crowd. It identifies all manner of options and allergens including wheat-free and lactose-free (no GF though!)
V, VG, GF - Im Herzen Afrikas (Gutleutstr. 13) The concept is simple, the food tasty and gluten-free flat breads are available.
V, VG, GF - Vevay (Neue Mainzer Str. 20) Vegetarian restaurant and predominantly vegan too. The menu clearly denotes other allergens. Very close to the New Opera house and city theatre on Willy-Brandt-Platz.
V, GF - Pizzeria Scicilia (Niedenau 9) on request this tiny, basic pizza place serves up excellent, gluten-free, pizza. Close to the Bahnhofsviertal however it is slightly off the beaten track.
In the city centre and old town
V, VG, GF- Mainkai (Mainkai 15) is just south of the Cathedral in the old town. It's known for it's breakfasts and offers plenty of choice for vegetarians and also some gluten-free and vegan options. At the weekend reserving a table for breakfast is advised!
V, VG, GF - Metropol Cafe (Weckmarkt 13-15) is right behind the Cathedral. Plenty of seating inside and on a sunny day it has a lovely garden to relax in. Metropol is popular with the locals at the weekend and has the option of a vegan breakfast. Vegetarian and vegan options are also available at other times of the day.
V, GF - Baltique (Heiligkreuzgasse 31 ) is a pancake cafe offering gluten free buckwheat pancakes in both savoury and sweet varieties. It's close to Konstablerwache and has a late kitchen (22:30).
V, VG, GF - Viapiano (Goetheplatz 1-3) in the heart of the city centre, this might be a chain restaurant but it clearly identifies vegan, vegetarian and gluten-free options on it's menu.
V - Leib & Seele (Kornmarkt 11) this is the staple "go to" restaurant if you want to try traditional German fair but need to cater for vegetarians in the group too.
V, VG - Lebegesund (Kleinmarkthalle) A market stall perfect for lunch tucked away at the back of the Kleinmarkthalle. Lebegesund offers a variety of vegan dishes to take away.
V, VG - Langosch am Main (Fahrgasse 3) Lots of vegan options in this laid back, kinda restaurant come bar. Nice area of outdoor seating too.