Document and Entity Information - shares
6 Months Ended
Jun. 30, 2019
Jul. 31, 2019
Document And Entity Information [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 30, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q2  
Trading Symbol GTHX  
Entity Registrant Name G1 THERAPEUTICS, INC.  
Entity Central Index Key 0001560241  
Current Fiscal Year End Date --12-31  
Title of 12(b) Security Common Stock, par value $0.0001 per share  
Security Exchange Name NASDAQ  
Entity Incorporation, State or Country Code DE  
Entity Filer Category Large Accelerated Filer  
Entity Common Stock, Shares Outstanding   37,522,593
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Current Reporting Status Yes    
Entity Shell Company false  
Entity File Number 001-38096  
Entity Address, Address Line One 79 T.W. Alexander Drive 4501 Research Commons  
Entity Address, Address Line Two Suite 100  
Entity Address, City or Town Research Triangle Park  
Entity Address, State or Province NC  
Entity Address, Postal Zip Code 27709  
City Area Code (919)  
Local Phone Number 213-9835  
Entity Tax Identification Number 26-3648180  
Document Quarterly Report true  
Document Transition Report false  
Entity Interactive Data Current Yes