I think dtd file associated in your tbx is not compatible with that of Globalsight. Globalsight uses TBXcdv04.dtd.
It seems Globalsight cannot parse xlsx file when import glossaries. But I'm sure that glossaries in xls are acceptable. What you need to do is correct your column headers. If you only have terms in source and target locale in your glossary, use the same language names defined in termbase as column header.
Other term attributes available are "Concept Domain","Concept Definition","Concept Source","Term Usage","Term Type","Term Definition","Term Example
". And attributes in double quotes should be the column headers.