Much of the ex-USSR is, at least on paper, quite twitchy about using GPS devices; there was a well-publicized case in Russia in 1997 where an American engineer was (briefly) imprisoned for accidentally using one near a military area.
That said, that was almost twenty years ago, and GPS in smartphones has become ubiquitous since then. Importing both dedicated GPS navigators and mobile phones into the Eurasian Customs Union, which includes Belarus, is now explicitly allowed:
the following goods and equipment may be freely imported into Russia and Custom Union and exported from Russia and Custom Union:
- mobile telephones, modems
- navigators ( GPS)
However, even in that old case, it's not the GPS itself that's problematic, but using it in a "manner which is determined to compromise Russian National Security". And Belarus, being a rather nasty police state, is the kind of place where "compromising national security" is anything that those in power want it to be.
What this boils down to is that if the militsia catch you snooping around a military base, you're in a world of trouble regardless of whether you've got a smartphone or GPS. But walking around with a smartphone is a lot more 'normal' than a dedicated GPS device, so at least you'd have some plausible deniability for having one with you.