We have traveled pretty extensively through Mexico in the past (not so the last couple years), and never had an issue with guitars and Customs. If you have a concern because the GSmini was made in Mexico, take your receipt along with you. Customs would only have an interest in it if they thought you were trying to bring in more stuff than you're allotted.
Regarding the Canada issue, it is best to have a list of equipment (instruments, cameras, pretty much any electronic devices that they could have concern that you are trying to re-sell in Canada) in triplicate. Keep one for yourself, leave one when crossing the border, present the other when coming back to show that you are returning with the same equipment.
Best wishes for a fun vacation,
Jim