Hi WP; welcome to LTG and congratulations on your Guild amp. That might the the rarest of the 'Thunder 1' series; member matsickma previously ID'd that model as having been marketed as a bass/practice amp. Like all of its Thunder 1 siblings, it runs on a pair of 12AX7s in the preamp, two 6GW8s output, and a 6CA4 rectifier ... good for 12-14 watts. The circuit may also vary from the standard schematic with some capacitor and resistor values changed to suppress higher frequencies; I'd expect this amp to be a Mr. Cleanjeans.
If you get a chance, a shot of the back would be nice; if your speakers are original, then they are probably square, ceramic-magnet CTS/Chicago Telephone Supply indicated by EIA code # 137-XXX. Some of these amps were fitted with speakers that had plastic bell covers to make them look like alnico speakers; the most collectible still have a red, stick-on 'Guild' logo tag on the cover.
The 6GW8s have been out of production for quite some time; all that's out there is the occasional new-old-stock and used tubes:
eBay 6GW8 search results. With moderate use, all vacuum tubes can last a lifetime ... or not. Don't know if you have backups but if/when you want them, this will be what you're looking at. There are no drop-in substitutes for the 6GW8 ... not even its cousin the 6BM8 and, due to the 6GW8's internal resistance, changing to another tube type for output means changing the output transformer too.
If you still have the original multi-section can capacitor in there and you want to upgrade the amp's performance, it's hard to over-emphasize the importance of fresh caps to having the amp reach it's highest potential; after that, a trip to the speaker store will also do wonders for the amp's tone. Anyway .. welcome to LTG!
