v3.26.1
Cover - USD ($)
 12 Months Ended
Dec. 31, 2025
Mar. 26, 2026
Jun. 30, 2025
Document Information [Line Items]      
Document Type 10-K    
Document Annual Report true    
Document Transition Report false    
Document Financial Statement Error Correction [Flag] false    
Entity Interactive Data Current Yes    
ICFR Auditor Attestation Flag false    
Amendment Flag false    
Document Period End Date Dec. 31, 2025    
Document Fiscal Year Focus 2025    
Document Fiscal Period Focus FY    
Entity Registrant Name GP-Act III Acquisition Corp.    
Entity Central Index Key 0001834526    
Entity File Number 001-42046    
Entity Tax Identification Number 00-0000000    
Entity Incorporation, State or Country Code E9    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Shell Company true    
Entity Filer Category Non-accelerated Filer    
Entity Small Business true    
Entity Emerging Growth Company true    
Entity Ex Transition Period false    
Entity Public Float     $ 303,312,500
Entity Address, Address Line One 300 Park Avenue, 2nd Floor    
Entity Address, City or Town New York    
Entity Address, Postal Zip Code 10022    
Entity Address, State or Province NY    
City Area Code (212)    
Local Phone Number 430-4340    
Units, each consisting of one Class A ordinary share and one-half of one redeemable warrant      
Document Information [Line Items]      
Title of 12(b) Security Units, each consisting of one Class A ordinary share and one-half of one redeemable warrant    
Trading Symbol GPATU    
Security Exchange Name NASDAQ    
Class A ordinary shares, par value $0.0001 per share      
Document Information [Line Items]      
Title of 12(b) Security Class A ordinary shares, par value $0.0001 per share    
Trading Symbol GPAT    
Security Exchange Name NASDAQ    
Redeemable warrants, each whole warrant exercisable for one Class A ordinary share at an exercise price of $11.50 per share      
Document Information [Line Items]      
Title of 12(b) Security Redeemable warrants, each whole warrant exercisable for one Class A ordinary share at an exercise price of $11.50 per share    
Trading Symbol GPATW    
Security Exchange Name NASDAQ    
Class A Ordinary Shares      
Document Information [Line Items]      
Entity Common Stock, Shares Outstanding   28,750,000  
Class B Ordinary Shares      
Document Information [Line Items]      
Entity Common Stock, Shares Outstanding   7,187,500