Please do realize the Bladder is a muscle. GBS can profoundly affect muscle and nerve functionality. You also didn't ID what your age is but getting older also impacts proper bladder function as well.
I highly suggest you see a Urologist for a consult. Be frank in how things were prior to the GBS as opposed to what you are experiencing now and ask to have a work up done to try to better ID the nature of the problem along with any treatment or rehab recommendations moving forward. Depending on the degree of impact you may have had from whatever type of GBS you were diagnosed with you might benefit from Bladder PT (yes, there really is such a thing even though it is highly specialized and as weird as it probably sounds).
A proper Eval needs to be done to sort out where the problem source might lie. Call your Primary MD to see what local Urologists he refers patients to and ask if he/she knows of any who have ever evaluated patients who have had GBS. Do your own research locally as well via online or phoning URO offices to see if any have had patients who have also had GBS. Check with your Health Insurer first before making any appointment to be seen as you may require your Primary MD to refer you first and the URO may not be within your Primary Medical Group's Specialty list and/or the URO may need to be In Network if you have PPO health coverage to prevent having to pay a far higher coinsurance, deductible etc. Call your Health Insurer's Customer Service Dept to find out if a specific Urologist is In Network for the type of coverage you have.