Here are some ideas you can decide what works best for you
Have a guild lottery once a week where half of the funds go to gbank half go to the winner (there used to be addons for this idk if there still are)
Do some old wotlk / cata raids to get titles / mounts (you should hardly have any deaths at all and % of all the gold looted each person goes to gbank) And if you can sell some of these titles or mounts to people you will get no where near the amount but you can still make a little bit
If you have someone you trust who plays the auction house and has multiple toons, give them access to the guild bank as the "banker" and allow them to spend / invest your guild gold to make more gold. You can allow him to keep an amount of the profit to motivate him/her to make more gold or give that player unlimited repairs.
You can help each guild member get more individually wealthy and instead of offering guild repairs / flasks you divide up all the gold the guild bank makes in the week through gold looted / challenges and /roll for BoE and recipes. Direct them to gold making blogs such as
http://wowgoldmadeeasy.blogspot.ca/