It appears that the only way to produce .gsm logos in a specific LogoManager format would be to use the original program itself, as there is probably no substitute for that. It can open and process Nokia logos (.nlm) as well, so seems to be ideal choice for this type of conversion.
Convert Nokia Logo file image format to Nokia LogoManager format.
We found one software available in the database for this conversion.
Microsoft Windows software - convert nlm to gsm on Windows
A utility used to edit operator logos on Nokia phones
Convert from nlm file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.