If you have loaded a GEDCOM for your Ancestors tab, and have found a person with just a first name, having that name recorded as their surname in GMPro, this is neither a GEDCOM nor a GMPro issue, but an issue from your family tree where you only have a single word entered for the person's name.
This can be easily resolved by entering a placeholder surname, five underscores _____ clearly shows that the missing name is not known. You can either edit manually, or reimport an updated GEDCOM to correct the names in your ancestors list, but as long as none of your assigned MRCA names have changed, you should not "lose" anything by doing so.
See also Options App Settings for further information
If you are missing some ancestors after loading the GEDCOM for a profile, then there are a number of possible causes.
An ancestor is not correctly connected to their parents, or has multiple parents and biological or preferred parents have not been set.
It's possible that your missing ancestor has been marked as “Private” or “Invisible” within your program and so hasn’t been exported as expected.
An ancestor marked as “Invisible” is unlikely to have any of their ancestors in your list.
If you have people named “Private” in your ancestor list, then you have marked that person as Private in your database, or you have done something at the time of exporting your Gedcom that has marked them as Private.
Check your program for the cause, correct it, and re-export your GEDCOM, paying attention to the privacy settings to ensure that names and dates are exported for living people, and re-load that new GEDCOM into Genome Mate Pro.
You can also manually add in the missing ancestors. This works well if you only have a handful of missing names; if you are missing more than 5 or 6, it may be easier to fix and re-export and re-import your GEDCOM.