You should ask the manufacturer or a dealer. Typically, you can't substitute a 6SN7GT in a circuit designed for the 6SN7GTB. The plate voltage is higher in the GTB (450V) vs. 350V for a GT, so the biasing would be different.
If a modern 6SN7GT like the Chinese Shuguang, yes, you can as even though they are marked as GTs, they are spec'ed more like GTBs and we've been using them that way for over 20 years with no worries. Don't do that with any of the boutique Chinese tubes like the Psvane- they are actual GTs.
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