Got this board myself, so went and asked Biostar support about ECC memory support. This is their reply
Dear Sir,
Regarding to your question,
X370GT5 specification does not support ECC memory,
however,we tested below ECC memory able to get display and run. (Note : Does not support ECC special function).
APACER DDR4 78.B1GM4.4020B 4GB 2133 MHz 1.2V
APACER DDR4 78.B1GM4.AF10B 8GB 2133 MHz 1.2V
HYNIX DDR4 HMA41GU7AFR8N-TF 8GB 2133 MHz 1.2V
Below is the full reply
Just wanted to share, in case someone was wondering about the ECC support