In Mexico, using a lawyer to try to get firearms is just paying someone to suck money out of you. The SEDENA (Secretaria de Defensa Nacional -- the Mexican Army) has a low opinion of lawyers and especially civilian lawyers "helping" a foreigner. Try to avoid them.
Find out the laws pertaining to firearms ownership and then try to find a local Club. This can be annoying as the Clubs tend to be less than welcoming to outsiders. Then try to find a local gun you can register and join the Club with. Once you are in, you are "in". You will make many contacts and learn the ropes to gun ownership in Guatemala. You won't find many Americans or Canadians in local gun clubs and once you get yourself in, I advise against trying to "import" foreign friends and friends of friends into the Clubs because that will probably work against you. Let those people do it on their own. Trust me on this.
In Mexico, guns cost more than in the U.S. and they are harder to get. Be that as it may, I have quite a few in Mexico as does my wife as well as transport permits that cover the entire country. So it can be done. I should mention that all the people you will be dealing with in the gun registration system are not going to speak English and (at least in Mexico) taking a translator with you is the kiss of death. When that Army Officer is asking you in Spanish (even if he secretly speaks English) "why do you want to own a gun?" he's asking you, not some translator guy or girl and will probably take it poorly. Try to speak Spanish as well as possible. If you have a Guatemalan wife or husband that would help a lot too probably but it's better if you do you own talking.
Speaking from the Mexican point of view -- which has stricter gun laws than Guatemala I think -- it absolutely can be done but it certainly isn't easy to do it. I would not count on being able to import guns from the U.S. legally. And a lawyer telling you that you can is probably just sucking money on a false promise. That's my opinion. Someone actually doing it would certainly prove me wrong.
Mexico has it's own firearms website under the name Mexico Armado. It covers in depth Mexican firearms laws, registry problems and offers a used firearm exchange as well. It is 100% in Spanish. It would not surprise me if a similar site exists in Guatemala. I do know that the current group running IPSC Mexico is looking at forming a Central American shooting circle comprising Mexico, Guatemala, Costa Rica, Panama, Honduras and a couple of other countries as well. I did an online Zoom interview for the Mexican Federacion de Tiro (FEMETI) over a year ago where we discuss this and the FEMETI group continues to post interviews from IPSC Clubs and regions throughout Central America.
So it's all happening. But it's happening in Spanish and probably that's how you'll have to search the information out -- in Spanish. Keep me informed as to how it's working on finding information about firearms ownership in Guatemala and if I find out anything myself I'll post it here for you.