So you used to use the same Bell phone line for both businesses but now have added VoIP from CanQuest to give you a new number.
The simplest solution would be plug a separate analog phone into the Grandstream. If you really want to use the Gigaset for both, you should stop using and advertising your new phone number as you may need to change providers.
While the website is very terse, it looks like CanQuest provides their own devices and offers installation services. This is not an appropriate VoIP service for your Gigaset. With only one analog phone port, you cannot connect both Bell and CanQuest to your Gigaset.
You should look for a bring-your-own-device VoIP provider instead. Ask CanQuest if they offer bring-your-own-device service. If they do, they will give you SIP credentials (username, password) along with the hostname of their VoIP server. If they do not, the simplest solution may be to abandon their service and to choose a new provider. If your number is already in heavy use, you can pay your new provider to port your phone number away from CanQuest.
Some good choices would be voip.ms, Anveo or Callcentric. Note that these services charge by the minute for outgoing calls. Incoming is also per minute, or alternatively unlimited use incoming plans for businesses at Anveo and Callcentric. Their rates are in USA dollars. You should be able to spend less with these providers if you have less than 1000 minutes per month of usage.
With bring-your-own-device VoIP service, the customer is fully responsible for configuring their telephone. You will have to log into your Gigaset's web interface to enter your SIP credentials and the VoIP server hostname. Callcentric has a webpage with step-by-step instructions for your Gigaset: