Well, the easy answer would be 'yes', though that's not entirely true.
It's possible to create an entire map in GUI (I still use GUI, even though I know JASS) and JASS functions really aren't needed in a Hero Castle Defense.
The problem here is that you need to use Custom scripts (GUI actions for JASS functions), so basically you're using JASS withing GUI triggers.
If you're not entirely sure what I mean, I'm talking about removing memory leaks.
Like this trigger:
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Set Location = Center of SomeRegion
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Unit - Create unit at Location
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Custom script: call RemoveLocation(udg_Location)
The last action is a custom script, which is JASS - though it is used in it's GUI-form.
Those tiny custom scripts (used only to remove memory leaks) are the only JASS-parts you need.
If you do not know what memory leaks are, or you wish more information on how to remove them or their background, there are plenty of tutorials here on the hive.
You
have to learn these very tiny codes of JASS to script properly (about 3 codes, all with the same structure).
And I also suggest using loops, preset tables and combining as many triggers with about the same event as possible.