Question

Do prefixes & suffixes have antonyms? As in, is it possible for a prefix or suffix to not have an antonym?

Example

Google defines "-gon" as:

-gon combining form in nouns denoting plane figures with a specified number of angles.

I can't find any antonyms for this suffix.

I've observed that prefixes may have antonyms. For example, post being the opposite for pre. But is this considered to be an antonym? If so, can suffixes also have antonyms?

Some prefixes and suffixes do have antonymous counterparts—sometimes multiple, while others don't have any (even though it would be useful if they did). "May have antonyms" is a good way to put it, although I hesitate to call them antonyms as they aren't technically words...

antonym : a word of opposite meaning The usual antonym of good is bad. —Merriam-Webster

But in answer to your final question, I believe there are suffixes that may pair antonymously...

-ful and -less as in thoughtful and thoughtless

You might notice that the prefixes and suffixes that may have antonymous counterparts are often complementary and not gradable—they have to do with binary extremes. Perhaps that's why you can't find one for -gon.