Is it possible to post job ads somewhere at GIS SE?
I have looked many places, but don't seem to find anything.
There is a Community Wiki dealing with GIS jobs on the Main site if that is what you are looking for.
The Q/A format of the site makes it inappropriate to advertise jobs directly in questions either here on meta or the main site. That being said, I personally feel it would be ok to drop a line in chat (as long as it is done in good taste, not overtly spam, and only done infrequently).
Also the main site has a separate place where one can post job listings for a fee.
Another way is to leave items/links in your profile page to your company or job postings. Of course the more questions and answers you ask the more people will visit your profile page!
Click the Advertising info link which appears at the bottom of every screen:
For sales and order support pertaining to Stack Overflow Careers related services, to buy job listings, or to access our candidate search database, you can always email us at firstname.lastname@example.org.
If you wish to advertise on the Stack Exchange network then the way that you can do that is via the Advertise With Us button at the bottom of every page.
If you are seeking to employ/contract someone then the Developer Jobs button at the bottom of every page may be of interest.
On the other hand, if individual users advertise their contact details on their user cards then by visiting them you may be able to find someone willing to undertake a job or an assignment for you, and the contact details necessary for you to contact them offline.