Access will be available to HTA members "at a preferential rate" for an unlimited number of users for an annual subscription fee.
The GROW Package has 90 haf hour training modules on retail, customer service, horticulture, health and safety and food hygiene.
The HTA launched an 'academy' last year which also provides eLearning, workbook or workshop training in customer service, visual merchandising and plant area care.
HTA chief executive Carol Paris said: "We are delighted to be able to announce this partnership to provide greater access to training for all of our member businesses. Working alongside our HTA Academy resource this will really help to provide the sector with a suite of options."
GCA chief executive Iain Wylie said: "This a fantastic opportunity for the GCA and HTA to collaborate to bring benefits to our industry. GROW as existing users will know is a great resource to deliver staff training. The flexibility offered by GROW helps the individual users and their garden centre employers business."
The GCA no longer has an office at HTA headquarters, after moving in in 2012, but has access to a meeting room. The GCA is believed to be looking to set up a new office in the Midlands.