dirs = [ name for name in os.listdir(--Directory--)) if os.path.isdir(os.path.join(str(--Directory--), name))]
for d in dirs:
d = str(d)
d = d.upper()
if d.find(".GDB") == -1:
outString = outString + str(d) + "; "
Methods like this are very helpful when you need to use tool validation. It allows you to quickly list all data in a directory, so users can select what they need.
You'll notice that the script has a find(). This is used because file geodatabases are essentially folders and therefore need to be excluded from the generated list.