Here is their Webite link http://www.thegardenersguild.co.uk/
And Gary is also a member of LJN
The PGG seems cheap to join but a little worried about the joining requirements as it implies rather grandly the need for 5 years full-time practical working experience in historic, heritage or botanic gardens etc... Could give the wrong impression?
So, still no one replies about benefits/drawbacks, experience.
They both can't have many members then!?
Joining any kind of organisation does not instantly make you more qualified or a better business than you are, prior to joining.
If you already have undertaken training and gained qualifications then LJN provides you with the facility to upload copies of any certificates or opportunity to list anything that a client might use as a benchmark when choosing you over any other land-based contractor.
If you use this site regularly in conjunction with the voluntary customer charter, it can go along way towards building an independently strong brand on which your client may gain trust.
The PGG are much more geared for estate gardeners, for example if searching for work through 'greycoat placements' or similar for full time contracts on large gardens / estates, PGG membership is a sought after accreditation.