What to upgrade to?

Sanders ESL's with his Magtech amp! I'd leave your TT system alone!

If you highly value what the Gamma Summits do well, and that is a substantial list, tread carefully replacing them. They are an excellent speaker and don't come up for sale very often. There is nothing wrong with trying to find something better, but the grass may not be greener. I would hold back selling them until you are certain you are making the right decision. 

Do you listen to mono lps?

I changed from SS to tube preamp and tube amp: thought I was 'done', then joined the forum, ... learned about the real difference playing mono lps with a mono cartridge has over playing mono with stereo cartridge (true), and, I wanted to try a longer arm. Not easy.

Ended up going for a large plinth two arm setup, 9" arm with mono cartridge ready to go, and a 12.5" arm with stereo cartridge ready to go. 9" arm has removable headshell, to use alternate stereo cartridges.

you need an easy way to pick which arm/cartridge. I didn't think about that, fortunately my old McIntosh mx110z tube preamp has phono 1 and phono 2, couldn't be easier, just got lucky.

I got newer tools for lp setup, accessories, and, I found a cleaning/drying system that makes a huge difference for my old lps playing quietly.

Now I think I am done. The lubricant in my reel to reel player and cd player is drying out, I know need to play them just to keep them limber.

I have about 10k in a complete turntable and my speakers retailed for 12k about 5 years. Both I could live with in retirement and be quite happy.

Congratulations! You have done it. Built a system you really enjoy. You should be proud of yourself. Way to go!

But I have $15k - $20k to spend if I want to on either upgrading my turntable or speakers. What would you upgrade? Turntable or speakers?

Wait- what??!?! I thought you just said.... and I said,... never mind.

This is where I get to put in my plug for how people are so totally missing the boat leaving so much performance on the table, sound they never in their lives imagined was right there in front of them just waiting to be unleashed and revealed. But no, instead of that they run out and spend money replacing perfectly good stuff they already had missing a golden opportunity to go so far beyond they could never even believe until they do it. And maybe not even then.

Save your money, bob. Tweaks are the answer. If you spent half of what you're talking about on stuff I know about you would have ten times the sound you could get with any combination of speakers and turntable you're considering now. Ten times. And I'm being conservative.