fix: Payload marshaling in gmp message generation

[rbajollari]

Description

closes: #XXXX

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[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (66acbcd) 15.62% compared to head (b9043b9) 16.10%.
Report is 4 commits behind head on main.

Files	Patch %	Lines
x/gmp/keeper/keeper.go	63.63%	3 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #343      +/-   ##
==========================================
+ Coverage   15.62%   16.10%   +0.48%     
==========================================
  Files          78       78              
  Lines       19225    18765     -460     
==========================================
+ Hits         3003     3022      +19     
+ Misses      16069    15590     -479     
  Partials      153      153              

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[adamewozniak]

lgtm! @rbajollari can you add an upgrade handler in this PR?